BIOLOGY 
LIBRARY 


GENEEAL  ZOOLOGY 


BY 

A.   S.   PEARSE 


f 


NEW  YORK 
HENRY  HOLT  AND  COMPANY 

1917 


"R3.T 

BIOLOGY 

LIBRARY 

G 


COPYBIQHT,    1917 
BT 

HENRY  HOLT  AND  COMPANY 


T  H    K     MAPLE     PRESS     YORK.     PA 


WHO,   THOUGH  WHOLLY   UNSCIENTIFIC, 

HAS  LOYALLY  DEVOTED  FIFTEEN 

YEARS  TO    THE  ADVANCEMENT 

OF  SCIENCE 


376237 


PREFACE 

This  book  has  been  written  to  be  read  by  students  of 
eighteen  to  twenty  years  of  age.  With  this  in  mind  the 
.chapters  have  been  made  short,  and  an  attempt  has  been 
made  to  enliven  the  text  by  interpolating  a  considerable 
number  of  illustrations  that  lean  more  toward  natural 
history  than  anatomy. 

The  purpose  of  the  book  is  to  give  a  general  survey  of 
the  important  points  relating  to  the  chief  groups  of  animals. 
The  first  four  chapters  deal  largely  with  generalities  and 
may  well  follow  chapters  which  come  later  if  such  an  ar- 
rangement is  more  expedient  for  any  particular  course. 
An  important  place  is  given  to  insects  because  they  afford 
the  most  available  material  for  collecting,  classification, 
and  dissection — students  can  acquire  a  variety  of  first-hand 
knowledge  from  them  as  from  no  other  group  of  animals. 

The  writer  is  convinced  that  students  of  biology  get  the 
best  training  in  scientific  methods  of  thought  from  practical 
work,  and  that  it  should  constitute  the  chief  part  of  any 
course;  also  that  a  text-book  should  not  repeat  what  is 
observed  in  laboratory  or  field,  but  supplement  and  general- 
ize upon  such  information.  The  book  ought,  therefore, 
to  contain  more  of  natural  history  and  general  biological 
theory  than  anatomy.  Some  chapters  in  the  present  work 
(e.g.,  X)  are  perhaps  ,  without  much  unity  or  apparent 
purpose,  and  are  intended  primarily  to  serve  for  reference 
in  connection  with  laboratory  or  field  studies. 

Anyone  who  teaches  is  stimulated  by  association  with 
great  teachers — men  of  originality  who  are  scientific,  yet 
human,  and  who  are  ever  willing  to  help  or  encourage 
students.  Though  this  book  intends  to  be  original,  the 
writer  is  fully  aware  that  he  has  consciously  and  uncon- 
sciously used  the  ideas  of  others.  Foremost  among  those 


vi  PREFACE 

who  have  thus  unwittingly  contributed  are:  G.  H.  Parker, 
W.  C.  Curtis,  and  M.  F.  Guyer. 

In  the  preparation  of  the  book  the  writer  has  been  under 
obligation  to  a  number  of  persons,  and  it  is  a  pleasure  to 
acknowledge  the  debt.  Professor  M.  F.  Guyer  read  the 
manuscript  for  chapters  I  to  V,  XXVII,  XXVIII,  XXX, 
and  helped  in  other  ways;  Professor  George  Wagner,  read 
chapters  XI  to  XX;  Professor  W.  S.  Marshall,  V  to  XI; 
Professor  W.  J.  Meek,  XXVII;  Dr.  John  N.  Lowe,  I  to 
IV;  Miss  G.  M.  White,  XXI  and  XXII;  Messrs.  A.  R. 
Cahn  and  T.  C.  Nelson,  XXV  and  XX  respectively.  Dr. 
A.  G.  Ruthven  and  Professor  E.  C.  Case,  of  the  University 
of  Michigan,  read  chapters  XXIII,  XXIV,  XXVI  and 
XXIX.  Most  of  all  I  am  indebted  to  Miss  Hattie  J. 
Wakeman,  who  drew  all  the  original  figures  but  two.  Fig. 
46  was  drawn  by  Lydia  Wakeman,  and  Mr.  A.  R.  Cahn 
furnished  the  prints  for  Fig.  103. 

UNIVERSITY  OP  WISCONSIN, 
March  1,  1917. 


CONTENTS 

CHAPTER  PAGE 

I.  INTRODUCTION 3 

II.  BASIS  FOR  THE  CLASSIFICATION  OF  ANIMALS  .    .  15 

III.  LIFE  AND  LIVING  THINGS 25 

IV.  CELLS 37 

V.  PHYLUM  ARTHROPODA,  CLASS  CRUSTACEA,  LAW  OF 

BIOGENESIS      . 45 

VI.  PHYLUM    ARTHROPODA,    CLASSES — ONYCOPHORA, 

MYRIAPODA,  INSECTA 63 

VII.  ORDER  ORTHOPTERA,  THE  RED-LEGGED  LOCUST  .     76 

VIII.  THE  RED-LEGGED  LOCUST 84 

IX.  PHYLUM  ARTHROPODA,  CLASS  INSECTA    ....     95 

X.  PHYLUM  ARTHROPODA,  CLASS  INSECTA     .....    106 

XI.  PHYLUM  ARTHROPODA,  CLASS  ARACHNID  A   .    .    .123 

XII.  PHYLUM  PROTOZOA 134 

XIII.  THE    ORIGIN    AND    CHARACTERISTICS    OF    THE 
METAZOA 147 

XIV.  PHYLUM  PORIFERA — SPONGES 157 

XV.  PHYLUM  COELENTERATA;  PHYLUM  CTENOPHORA   .    163 

XVI.  PHYLUM  PLATYHELMIA 174 

XVII.  PHYLUM    NEMATOIDEA;    PHYLA:  ROTIFERA, 

BRACHIOPODA,  BRYOZOA 183 

XVIII.  PHYLUM  ECHINODERMATA    .    .    .    , 192 

XIX.  PHYLUM  ANNELIDA 203 

XX.  PHYLUM  MOLLUSCA  .    .    .   • 213 

XXI.  PHYLUM  CHORDATA 224 

XXII.  PHYLUM  CHORDATA,  CLASS  PISCES 235 

XXIII.  PHYLUM  CHORDATA,  CLASS  AMPHIBIA 246 

XXIV.  PHYLUM  CHORDATA,  CLASS  REPTILIA 257 

XXV.  PHYLUM  CHORDATA,  CLASS  AVES 269 

XXVI.  PHYLUM  CHORDATA,  CLASS  MAMMALIA    ....   286 

XXVII.  MAN.     SPECIAL  FEATURES 297 

XXVIII.  MAN.     GENERAL  FEATURES 311 

XXIX.  ANIMALS  OF  THE  PAST 327 

XXX.  EVOLUTION  AND  HEREDITY 342 

INDEX 361 

vii 


GENERAL  ZOOLOGY 


CHAPTER  I 
THE  ABUNDANCE  OF  ANIMAL  LIFE 

'  Most  persons  do  not  realize  what  an  enormous  number  of 
animals  exist  on  the  earth,  and  what  a  variety  of  habitats 
they  occupy.  The  forest  not  only  harbors  monkeys, 
squirrels,  tree  frogs,  and  other  familiar  animals  which  are 
suited  to  such  a  situation,  but  also  supports  a  host  of 
minute  insects,  worms,  snails,  and  other  things  which 
escape  ordinary  notice.  Many  of  these  are  active  only  at 
night,  on  humid  days,  or  on  various  infrequent  occasions. 
The  fields  and  prairies  have  a  characteristic  fauna  of 
prairie-dogs,  grasshoppers,  antelope,  etc.  Lakes,  ponds, 
and  streams  swarm  with  aquatic  animals.  Particularly 
in  winter,  bodies  of  water  serve  as  refuges  for  many  animals 
which  may  be  found  elsewhere  during  warmer  seasons. 
Some  animals,  such  as  the  mole,  the  mole-cricket,  and  the 
earthworm,  pass  their  whole  lives  burrowing  in  the  soil,  and 
show  structural  adaptations  which  fit  them  particularly  for 
such  a  habitat. 

Unsuspected  residents  may  occur  in  all  sorts  of  situations 
and  in  countless  numbers.  Darwin  was  once  interested 
in  the  distribution  of  animals  and  plants  on  the  feet  of  birds. 
In  this  connection  he  tried  to  ascertain  how  many  living 
things  there  were  on  the  muddy  shore  of  a  little  puddle. 
From  three  spoonfuls  of  mud  he  raised  537  separate  plants. 
More  recently  one  of  the  investigators  for  the  United  States 
Department  of  Agriculture,  in  studying  the  food  of  birds, 
took  a  census  of  all  the  animal  and  plant  objects  in  a  space 
two  feet  square  and  as  deep  as  a  bird  might  scratch,  with 
the  following  result: 

Animal  Plant 

objects  objects 

Forest 112  194 

Meadow..  1254  3113 


'    —o^ 
4  GENERAL  ZOOLOGY 

Probably   most   readers   have   never   heard   of   nematode 
worms,  yet  Cobb  in  a  paper  on  these  animals  says: 

"Not  the  least  interesting  thing  about  nematodes  is  the  astounding 
variety  of  their  habitats.  They  occur  in  arid  deserts,  at  the  bottoms 
of  lakes  and  rivers,  in  the  waters  of  hot  springs  and  in  polar  seas  where 
the  temperature  is  constantly  below  the  freezing  point  of  fresh  water. 
They  were  thawed  out  alive  from  Antarctic  ice  in  the  far  south  by  mem- 
bers of  the  Shackleton  expedition.  ...  A  thimbleful  of  mud  from  the 
bottom  of  river  or  ocean  may  contain  hundreds  of  specimens.  The 
nematodes  from  a  10-acre  field,  if  arranged  in  single  file,  would  form  a 


FIG.  1. — A  nematode  worm  which  lives  on  the  roots  of  plants.     Greatly  mag- 
nified.    (From  Cobb,  Yearbook;  U.  S.  Department  of  Agriculture,  1914.) 

procession  long  enough  to  reach  around  the  world.  A  lump  of  soil  no 
larger  than  the  end  of  one's  thumb  may  contain  hundreds,  even  thou- 
sands of  nematodes,  and  yet  present  few  points  that  would  distinguish  it 
from  a  lump  of  soil  destitute  of  these  organisms.  ...  In  short,  if  all 
other  matter  in  the  universe  except  the  nematodes  were  swept  away, 
our  world  would  still  be  dimly  recognizable,  and  if,  as  disembodied 
spirits,  we  could  then  investigate  it,  we  should  find  its  mountains,  hills, 
vales,  rivers,  lakes,  and  oceans  represented  by  a  film  of  nematodes.  The 
location  of  towns  would  be  decipherable,  since  for  every  massing  of 
human  beings  there  would  be  corresponding  massing  of  certain  nema- 
todes. Trees  would  stand  in  ghostly  rows  representing  our  streets  and 
highways." 

Instances  of  this  kind  might  be  multiplied.  There  are 
animals  in  the  soil  which  even  exceed  the  nematodes  in 
numbers. 


THE  ABUNDANCE  OF  ANIMAL  LIFE  5 

DEFINITIONS 

Biology  is  the  branch  of  natural  science  which  includes 
all  studies  pertaining  to  living  things.  There  are  two  great 
subdivisions:  Botany,  which  deals  with  plants;  and  Zool- 
ogy, which  is  concerned  with  animals.  Zoology,  then,  is 
the  body  of  facts  and  theories  derived  from  the  scientific 
study  of  animals.  Our  science  is  founded  on  facts,  and 
over  this  superstructure  there  have  been  erected  a  number 
of  theories  which  are  intended  to  blend  zoology  into  a 
harmonious  whole.  We  must  be  scientific  in  our  study. 
This  is  the  hardest  task  a  zoologist  has  from  day  to  day. 
A  scientist's  aim  is  to  discover  the  truth  about  phenomena. 
A  real  scientist  formulates  his  conclusions  and  general  laws 
from  a  study  of  the  available  facts,  and  he  does  this  without 
prejudice,  or  superstition,  or  thought  of  self  interest.  Let 
us  be  scientific!  This  means  accurate  observation  and 
sound,  thoughtful  conclusions. 

On  account  of  the  amount  of  material  which  has  been 
•  accumulated  from  zoological  studies,  men's  interests  have 
been  divided  and  a  number  of  subsciences  established. 
Chief  among  these  are  the  following: 

Systematic  Zoology  deals  with  the  description  of  species 
and  their  classification  according  to  accepted  systems. 
About  522,400  species  have  been  described,  and  the  mere 
cataloguing  of  these  requires  considerable  work  by  special- 
ists. There  are  many  branches  of  systematic  zoology, 
such  as:  entomology,  relating  to  the  classification  of  insects ; 
ornithology,  birds;  conchology,  molluscs,  etc. 

Distributional  Zoology  has  two  aspects.  Under  Zoogeog- 
raphy are  included  facts  relating  to  the  present  arrange- 
ment of  animals  on  the  earth.  Five  great  realms  which 
possess  characteristic  animals  are  recognized  (Holoarctic, 
African,  Indo-Malayan,  Neotropical,  Australian),  and 
these  have  each  been  subdivided  into  smaller  parts. 
Paleozoology  deals  with  the  distribution  of  animals  during 


6  GENERAL  ZOOLOGY 

the  past,  as  shown  by  fossil  remains.  Zoogeography,  then, 
has  reference  to  space;  Paleozoology,  to  time. 

Morphology  is  the  science  of  structure.  Anatomy  deals 
with  dissection;  embryology,  with  the  changes  which  take 
place  as  individuals  develop  to  maturity;  histology,  with 
minute  anatomy  or  the  study  of  tissues;  pathology,  with  the 
structure  of  diseased  tissues;  neurology,  with  the  make  up 
of  nerves;  paleontology,  with  the  characteristics  of  fossil 
animals. 

Physiology  treats  of  the  functions  of  the  parts  of  animals 
— i.e.,  how  the  structures  which  make  up  the  mechanism  of 
any  animal  work.  As  under  morphology  there  are  a  num- 
ber of  subdivisions:  psychology  is  concerned  with  the  work- 
ing of  the  mind;  psychiatry,  with  the  operations  of  diseased 
or  abnormal  minds. 

Ecology  considers  the  relations  of  animals  to  their  en- 
vironment. Here  would  properly  come  such  topics  as  the 
relations  of  parasites  to  their  hosts,  colonial  habits,  the 
denizens  of  particular  habitats,  etc. 

Evolutionary  Zoology  relates  to  the  origin  and  descent  of 
species.  Heredity  is  concerned  with  how  characteristics 
are  transmitted  from  parent  to  offspring.  Eugenics  deals 
particularly  with  heredity  in  man,  with  the  aim  of  improv- 
ing the  human  race. 

SHORT  HISTORY  OF  ZOOLOGY 

If  we  are  going  to  be  zoologists,  even  in  a  small  way,  it  is 
fitting  that  we  should  know  something  about  those  who 
have  laid  the  foundations  of  our  science.  The  following 
ten  names  have,  therefore,  been  selected  to  serve  as  "  mile- 
stones of  zoological  progress." 

ARISTOTLE,  the  " Father  of  Natural  History,"  was  a 
Greek  scholar  who  lived  from  384  to  322  B.C.  He  wrote  in 
all  about  three  hundred  works  on  philosophy,  metaphys- 
ics, psychology,  and  rhetoric,  but  his  most  noteworthy 
writings  were  in  the  field  of  natural  history.  His  greatest 


HISTORY 


contribution  to  zoology  was  his  method  of  study.  He 
gathered  first-hand  knowledge  about  animals  and  presented 
his  facts  in  a  scientific  way.  Locy  says,  "the  influence  of 
Aristotle  was  in  the  right  direction.  He  made  a  direct 
appeal  to  nature  for  his  facts,  and  founded  his  Natural 
History  only  on  observation  of  the  structure,  physiology, 
and  development  of  animals."  Aristotle's  best  books,  the 
" History  of  Animals,"  the  " Parts  of  Animals,"  and  the 
" Generation  of  Animals,"  were  authoritative  for  twenty 
centuries.  Aristotle  led  an  active  public  life,  being  a 
student  of  Plato  and  a  teacher  of  Alexander  the  Great. 


FIG.  2. — Aristotle.      (From  Locy, 
Biology  and  Its  Makers.) 


FIG.  3. — Pliny.      (From  Locy, 
Biology  and  Its  Makers.) 


PLINY,  23-79  A.D.,  was  a  Roman  general  and  writer. 
He  wrote  thirty-seven  voluminous  and  uncritical  volumes 
on  natural  history.  His  works  were  filled  with  tales  of 
dragons,  gorgons,  and  other  fabled  monsters,  so  that  it 
was  impossible  to  separate  fact  from  fiction.  His  influence 
was  highly  detrimental  to  the  progress  of  zoological  thought, 
but  his  works,  nevertheless,  were  authoritative  during  the 
middle  ages. 

GALEN,  130-200  A.D.,  was  a  Greek  physician.  He  was 
the  great  anatomist  of  antiquity  and  his  clear  and  forceful 
descriptions  were  the  sole  anatomical  guides  in  the  medical 
schools  for  twelve  centuries  after  his  time.  He  was  a 
sound  thinker  and  accurate  observer.  Galen  probably 
never  actually  dissected  the  human  body,  but  wrote  his 


8 


GENERAL  ZOOLOGY 


excellent   works   from  studies   and   comparisons  of  other 
animals. 

During  the  Middle  Ages  there  was  no  progress  in  zoo- 
logical thought.  All  questions  were  referred  to  ancient 
authorities.  Matters  of  learning  were  almost  wholly  in 
the  hands  of  the  clergy,  who  deemed  everything  unworthy 
which  did  not  pertain  directly  to  religious  life.  The  ad- 


FIG.  4. — Galen.     (From  Locy, 
Biology  and  Its  Makers.) 


FIG.  5. — Vesalius.      (From  Locy, 
Biology  and  Its  Makers.) 


monition  to  "shun  the  world "  was  taken  all  too  seriously 
by  the  early  Christians.  There  was  stagnation  in  all 
fields  of  learning. 

VESALIUS  (1514-1564,  Belgian),  more  than  any  man, 
threw  off  the  yoke  of  tradition  and  the  respect  for  authority 
which  had  characterized  the  scholars  of  the  Middle  Ages 
and  opened  the  way  for  the  free  discovery  of  new  knowledge 
which  we  enjoy  today.  He  was  a  man  of  great  courage 
and  honesty.  Despite  the  superstitious  traditions  of  his 
time,  he  dissected  the  human  body.  His  eminent  book, 
the  "  Structure  of  the  Human  Body, "  and  his  direct  methods 


HISTORY 


9 


of  teaching  revolutionized  anatomical  methods.  His 
greatest  work,  however,  was  the  opening  of  men's  minds 
to  the  opportunities  for  scientific  progress  to  those  who 
would  make  observations  for  themselves  and  draw  their 
own  conclusions. 

HARVEY  (1578-1667,  English),  following  the  anatomical 
discoveries  of  Vesalius,  laid  the  foundations  of  our  modern 
methods  of  experimental  investigation  in  biological  science. 
His  great  work  was  the  demonstration  of  the  circulation 
of  the  blood,  an  epoch-making  discovery.  Before  his  time 
it  was  supposed  that  there  was  a  sort  of  an  ebb  and  flow 


FIG.  6. — Harvey.      (From  Locy,  Biology 
and  Its  Makers.) 


FIG    7. — Linnaeus.     (From  Locy, 
Biology  and  Its  Makers.) 


in  the  blood-vessels,  that  the  arteries  contained  blood  mixed 
with  animal  spirits  from  the  lungs,  and  that  the  veins  held 
the  crude  blood.  Harvey  showed  conclusively  that  the 
blood  traversed  a  regular  route  through  the  body.  This 
may  seem  like  a  simple  everyday  fact  at  the  present  time, 
but  in  the  fifteenth  century  Harvey's  assertions,  though 
they  were  finally  accepted,  excited  great  controversy  and 
astonished  the  whole  scientific  world. 

LINNAEUS    (1707-1778)    was    the    founder    of    modern 
systematic  zoology.     Before  his  time  species  were  rather 


10 


GENERAL  ZOOLOGY 


indefinitely  known  from  verbose  Latin  descriptions.  This 
great  Swedish  naturalist  inaugurated  our  present  binomial 
nomenclature,  giving  to  each  species  a  scientific  name  con- 
sisting of  two  words,  the  genus  and  species.  He  attempted 
to  describe  and  catalogue  all  species  of  plants  and  animals. 
In  the  tenth  edition  of  his  great  work,  the  Systema  Naturae, 
4236  species  of  animals  were  described.  The  general  classi- 
fication instituted  by  Linnaeus  was  not  very  satisfactory 

and  has  now  been  considerably 
amplified.  In  his  later  life  Lin- 
naeus was  a  professor  in  the 
University  of  Upsala  and  the 
foremost  figure  in  the  zoological 
world.  His  influence  made 
classification  and  naming  new 
species  the  most  studied  fields 
of  biology  for  some  time  after 
his  death. 

CUVIER  (1769-1832)  was  an 
eminent  educator,  being  direc- 
tor of  the  higher  institutions  of 
learning  under  Napoleon  Bona- 
parte. Early  in  life  he  con- 
ceived the  idea  of  making  a  very  comprehensive  study  of 
comparative  anatomy  and  was  so  successful  that  this  sub- 
ject became  the  leading  field  of  zoology  until  the  time  of 
Darwin.  He  also  made  very  thorough  studies  of  the  anat- 
omy of  all  the  chief  groups  of  animals  which  had  existed 
in  bygone  ages.  Cuvier  would  not  accept  the  idea  of  an 
evolution  of  animals,  but  believed  the  earth  had  been  re- 
populated  again  and  again  after  a  series  of  "catastro- 
phisms"  which  wiped  out  everything  alive. 

DARWIN  (1809-1882)  was  an  Englishman  who  received 
most  of  his  zoological  training  during  a  five-year  cruise  as 
naturalist  on  the  ship  Beagle,  which  made  a  voyage  around 
the  world.  After  his  return  to  England,  he  began  with 
great  care  to  accumulate  data  for  his  theory  of  evolution. 


FIQ.  8. — Cuvier.     (From  Locy, 
Biology  and  Its  Makers.} 


HISTORY 


11 


This  theory  is  well  set  forth  in  the  "  Origin  of  Species/' 
published  in  1859.  His  great  work  for  Zoology,  and  the 
world,  was  to  convince  scientific  men  that  there  had  been 
a  change  in  the  past — that  one  type  of  animal  had  evolved 
from  a  somewhat  different  preexisting  type.  His  theory 
involved  four  ideas:  (1)  More  animals  are  produced  than 
can  find  a  place  to  live  and  hence  there  is  a  "  struggle  for 
existence;"  (2)  qualities  tend  to  be  transmitted  unchanged 
from  parent  to  offspring;  but  (3)  there  is  always  slight  varia- 
tion which  may  make  one  animal  a  little  better  than  others 
of  its  own  species;  and  (4)  there  may  thus  be  a  " survival 
of  the  fittest"  in  the  struggle,  which  constitutes  the 


FIG.  9. — Darwin.     (From  Locy, 
Biology  and  Its  Makers.) 


FIG.  10. — Agassiz.     (From  Locy, 
Biology  and  Its  Makers.) 


' '  natural  selection ' ' 
improved   after 


through  which  Darwin  supposed  animals 
a  number  of  generations.  This  theory 
gave  a  wonderful  impetus  to  all  branches  of  scientific  work 
because  it  opened  a  vast  field  for  investigation  by  clearing 
men's  minds  of  the  idea  that  natural  phenomena  had  always 
been  and  always  would  be  as  they  are,  and  were,  therefore, 
not  open  to  experimental  study. 

AGASSIZ  (1807-1873),  though  a  Swiss  by  birth,  is  of 
particular  interest  to  all  American  zoologists.  He  was  a 
comparative  anatomist  of  the  school  of  Cuvier  who  came 


12 


GENERAL  ZOOLOGY 


to  America  when  a  young  man  and  introduced  the  scientific 
methods  of  Europe  into  this  country.  Though  he  was  the 
first  to  work  out  the  close  relation  between  the  fossil  record 
and  the  embryonic  stages  in  the  development  of  individual 
animals,  he  would  never  accept  his  discoveries  as  evidences 
of  evolution,  and  died  an  opponent  of  Darwin. 

HUXLEY  (1825-1895),  though  primarily  a  student  of 
fossil  animals,  did  a  great  work  along  general  zoological 
lines  for  all  English-speaking  people.  More  than  any  other 

zoologist  he  was  able  to  talk 
and  write  in  an  understand- 
able way  to  popular  audiences. 
His  contribution  to  zoological 
progress,  then,  was  to  popu- 
larize zoology  and  make 
general  biological  laws  mat- 
ters of  common  everyday 
knowledge. 

From  this  brief  historical 
survey  it  is  plain  that  zoology 
has  not  come  about  all  at 
once.  There  has  been  a  long 
struggle  to  accumulate  facts 

and  to  make  theories  to  fit  them.  No  one  nation,  nor 
class  of  people  has  done  the  good  work,  but  a  host  of 
fair-minded  energetic  workers  have  contributed  to  progress. 
One  of  the  fine  things  about  science  is  its  method  of  work. 
There  is  no  secrecy  nor  selfishness — only  desire  to  discover 
truth  at  any  cost.  For  science  men  have  died  of  fever 
during  explorations;  carried  on  life-long  struggles  with 
poverty,  superstition,  or  other  obstacles;  or  been  shunned 
by  their  fellows  for  advocating  new  ideas.  But  through 
the  courage  and  inspiration  of  such  workers  zoology  has 
gone  forward;  we  are  not  yet  at  the  end  of  our  discov- 
eries; new  things  are  flashing  into  the  light  of  knowledge 
almost  every  day. 


FIG.    11. — Huxley.      (From  Locy, 
Biology  and  Its  Makers.) 


CLASSIFICATION  13 

THE  CLASSIFICATION  OF  ANIMALS 

At  the  present  time  there  are  about  523,000  species  of 
animals  known,  with  perhaps  an  equal  number  yet  to  be 
described.  It  goes  without  saying  that  such  a  number  of 
distinct  kinds  requires  some  means  of  classification,  if  only 
for  convenience  in  cataloguing.  There  are  in  general  two 
systems  of  classification — natural  and  artificial.  The  first 
is  the  result  of  an  attempt  to  show  genetic,  or  blood,  rela- 
tionships; the  second  term  is  applied  to  any  system  of 
grouping  which  will  enable  one  to  bring  animals  together 
which  show  certain  points  of  similarity,  whether  such 
characteristics  are  of  fundamental  importance  or  not.  la 
times  past  the  barnacles  were  classed  with  molluscs  be- 
cause they  possessed  a  calcareous  shell,  but  when  their 
embryology  was  studied  it  was  discovered  that  all  the  early 
stages  were  like  those  of  crustaceans,  and  they  have  since 
been  classified  with  other  members  of  that  group — such  as 
crabs,  lobsters,  and  water-fleas.  The  former  classification 
for  the  barnacles  was  artificial,  the  latter,  it  is  believed,  is 
more  natural.  Systematic  zoologists  are  continually  striv- 
ing for  a  perfect  natural  classification. 

The  animal  kingdom  is  divided  into  smaller  and  smaller 
units  just  as  a  military  regiment  is  divided  into  battalions, 
companies,  platoons,  squads,  and  individuals.  The  largest 
groups  are  called  phyla,  these  are  subdivided  into  classes, 
etc.  In  going  from  larger  to  smaller,  the  groups  would  be 
arranged  as  follows :  Phylum,  Class,  Order,  Family,  Genus, 
Species. 

Each  species  of  animal  is  known  by  its  scientific  name, 
which  consists  of  the  name  of  its  genus  and  its  own  particu- 
lar species.  Taking  man  as  an  example,  we  may  classify 
him.  He  belongs  to  the  species  sapiens  and  the  genus 
Homo.  This  genus  also  includes  some  other  species  of 
men,  none  of  which  are  now  living.  The  scientific  name  of 
man  would  be,  Homo  sapiens  Linnaeus;  the  last  word 
showing  that  Linnaeus  first  described  this  particular  species. 


14  GENERAL  ZOOLOGY 

The  genus  Homo  is  grouped  with  a  number  of  others  con- 
taining ape-like  animals  in  the  family  Hominidse,  and  this 
family  in  turn  is  one  of  those  which  make  up  the  order 
Primates,  including  monkeys  of  all  kinds.  Primates  is  one 
of  the  orders  of  the  class  Mammalia,  which  includes  all 
animals  which  have  hair  and  suckle  their  young — such  as 
bats,  seals,  whales,  hoofed  animals,  cats,  dogs,  etc.  The 
class  Mammalia  is  one  of  half  a  dozen  in  the  phylum 
Chordata,  one  of  the  primary  divisions  of  the  animal 
kingdom. 


CHAPTER  II 
BASIS  FOR  THE   CLASSIFICATION  OF  ANIMALS 

If  one  is  going  to  classify  600,000  different  kinds  of 
animals,  it  will  be  necessary  to  have  some  general  plan  for 
making  large  divisions  which  will  include  a  considerable 
number  of  species  having  common  characteristics.  To 


FIG.   12. — A  bit  of  cork  showing  cells  as  represented  by  Hook,  the  discoverer  of 
cells.      (From  Locy,  Biology  and  Its  Makers.) 

make  a  natural  classification  the  seven  following  criteria 
have  been  found  of  most  value: 

1.  Body  Composed  of  One  Cell  or  of  Many. — All  living 
things  are  made  up  of  little  living  units,  the  cells  (Fig.  12), 
just  as  a  brick  house  is  made  up  of  bricks.  A  cell,  like  a 
brick,  may  thus  exist  as  part  of  a  structure  or  as  an  inde- 
pendent unit.  One  great  division  of  animals,  Protozoa,  is 
made  by  grouping  all  those  together  whose  bodies  are  com- 

15 


16 


GENERAL  ZOOLOGY 


posed  of  one  single  cell;  another,  Metazoa,  by  grouping 
together  those  whose  bodies  are  multicellular. 

2.  Diplo-  or  Triplo-blastic  (Fig.  13). — Animals  made  up 
of  many  cells  show  differences  in  arrangement.  In  the 
sponges  the  cells  do  not  form  layers  but  in  other  meta- 
zoans  they  form  definite  sheets.  We  may  have,  therefore, 


FIG.  13. — Sections  across  the  bodies  of  Hydra  and  Planaria  showing  diplo- 
blastic  and  triploblastic  structure.  In  the  first  there  are  two  layers  of  cells  be- 
tween the  digestive  cavity  and  the  outside;  in  the  second,  there  are  three. 

diploblastic  animals  with  two  layers  of  cells — the  ectoderm 
and  entoderm;  or  triploblastic  animals  with  three — ectoderm, 
mesoderm,  and  entoderm.  These  primitive  sheets  of  cells 
which  appear  in  the  development  of  most  Metazoa  are 
called  the  germ  layers. 

3.  Body   Metameric  or  Non-metameric   (Fig.    14). — The 
bodies  of  many  animals  show  a  condition  of  metamerism, 


FIG.  14. — A  slug  and  a  leech,  showing  the  absence  and  presence  of  metamer- 
ism. The  slug  has  no  subdivisions  in  the  body,  but  in  the  leech  the  body  is  made 
up  of  a  series  of  similar  parts. 

or  segmentation.  Similar  parts  are  arranged  one  after 
the  other  and  form  a  sort  of  a  chain.  Each  of  the  seg- 
ments has  a  more  or  less  complete  set  of  organs,  and  these 
are  repeated  in  the  successive  segments  of  the  body.  Man 


CLASSIFICATION  OF  ANIMALS  17 

shows  evidences  of  metamerism  in  the  arrangement  of  his 
ribs,  vertebrae,  spinal  nerves,  muscles,  etc.  The  common 
earthworm  is  a  better  example  for  its  body  is  composed  of 
a  chain  of  very  similar  metameres. 

4.  Symmetry  Radial  or  Bilateral  (Fig.  15). — Some  animals 
have  the  chief  organs  arranged  around  a  central  axis.  The 
form  is  usually  disc-like  or  spherical,  perhaps  also  with  pro- 
jecting arms.  In  such  radially  symmetrical  animals  there 
are  a  number  of  antimeres,  or  body  parts  which  might  be 
interchanged  without  destroying  the  original  symmetry, 
just  as  one  could  exchange  two  quarters  of  a  pie  without 


FIG.  15. — A   jelly-fish   and   a   turtle,    showing   radial   and   bilateral   symmetry 

respectively. 

spoiling  its  general  shape.  Jelly-fishes,  corals,  and  star- 
fishes are  examples.  Those  which  grow  fixed  to  some  ob- 
ject have  distal  (free)  and  proximal  (attached)  ends;  those 
which  move  freely  have  no  "head"  or  "tail"  end,  but 
possess  oral  (mouth)  and  aboral  surfaces.  In  bilaterally 
symmetrical  animals,  on  the  other  hand,  the  body  has  a 
front  or  anterior  end,  which  often  is  distinguished  as  a  head. 
The  opposite  end  is  the  posterior  or  hind  portion;  there  are 
right  and  left  sides;  a  dorsal  or  back,  and  a  ventral,  or  belly 
side.  Such  animals  are  symmetrical  on  two  sides  of  a  plane 
which  passes  down  the  middle  of  the  dorsal  and  ventral 
sides.  The  portions  on  either  side  of  this  plane  are  called 


18 


GENERAL  ZOOLOGY 


lateral.     Man  may  serve  as  an  example  of  a  bilaterally 
symmetrical  animal. 

5.  Body  Sac-like  or  a  Tube  Within  a  Tube,  i.e.,  Accelomate 
or  Ccelomate  (Fig.  16). — Some  animals,  like  the  fresh-water 


FIG.  16. — A  Hydra  and  the  anterior  end  of  an  earthworm.  Both  have  been 
split  lengthwise  so  that  the  inside  of  the  body  is  exposed.  The  Hydra  has  only 
one  cavity,  the  enteron,  or  digestive  cavity;  the  earthworm  has  another  cavity, 
the  ccelom,  between  the  digestive  tube  and  the  body  wall. 


FIG.  17. — Showing  endo-  and  exoskeletons.  The  bones  in  a  man's  leg  are 
surrounded  by  muscles;  the  skeleton  of  a  grasshopper's  leg  consists  of  tubes 
with  muscles  inside. 

hydra,  have  only  one  cavity  within  the  body  and  the  out- 
side wall  is  solid.  There  is  only  one  opening  into  the  cavity, 
the  mouth.  Other  animals,  like  the  earthworm,  have  a 


CLASSIFICATION  OF  ANIMALS  19 

tubular  digestive  system,  with  a  mouth  at  one  end  and  an 
anus  at  the  other.  Around  this  tube  there  is  a  fluid-filled 
space,  the  ccelom  or  body-cavity,  -which  intervenes  between 
it  and  the  solid  body  wall.  The  earthworm  is  again  a  good 
illustration  of  this  point. 

6.  Appendages. — Many  animals  possess  appendages  to 
the  body   proper   in   the   form   of   paddle-like  swimming 
organs,  fins,  jointed  legs,  etc. 

7.  The  Type  of  Skeleton  (Fig.  17). — Animals  may  have 
hard    structures    on    the    outside    (exoskeleton)    or    inside 
(endoskeletori)  of  the  body.     In  the  vertebrates  and  their 
relatives  the  skeleton  is  typically  of  the  latter  type  and 
its  central  axis  always  originates  as  a  rod-like  organ,  the 
notochordj  or  chorda,  which  extends  longitudinally  through 
the  body.     In  the  vertebrates  proper  the  chorda  is  re- 
placed during  early  development  by  bony  vertebrae  which 
form  the  spinal  column,  or  " backbone." 

Using  such  criteria,  it  is  possible  to  divide  the  animal 
kingdom  into  fifteen  great  groups,  or  phyla,  which  may  be 
identified  by  means  of  the  following  "key": 

KEY  TO  PHYLA  OF  ANIMALS* 

1  (2)  Body  composed  of  one  cell  or  a  colony  of  similar  cells;  mostly 

microscopic Phylum   I,   PROTOZOA. 

2  (1)  Body  composed  of  many  cells  arranged  in  tissues 3 

3  (24)  Body  non-metameric 4 

4  (11)  Radially  symmetrical.     Have  proximal  (attached)  and  distal 

(free)  ends,  or  have  oral  (mouth)  and  aboral  surfaces,  but  have 
no  head  or  tail.     Parts  of  body  radiate  around  a  chief  axis.  .  .5 

5  (6)  Body   filled   with   many   minute   pores;   no   definite   mouth. 

Sponges Phylum     II,     PORIFERA. 

6  (5)  Body  not  filled  with  pores ;  a  mouth  present 7 

7  (8)  Body  with  eight  rows  of  comb-like  plates  arranged  radially. 

Phylum  IV,  CTENOPHORA. 

8  (7)  Body  without  eight  rows  of  radially  arranged  paddle-plates. .  9 

9  (10)  Body  sac-like,  often  with  tentacles.     Polyps,  medusae,  jelly- 

fishes,    corals,    sea-pens.  ..  .Phylum   III,    CXELENTERATA. 

*  In  using  the  key  there  are  always  two  alternatives.  The  first  number 
is  to  be  compared  elsewhere  with  the  one  in  parenthesis  after  it. 


20  GENERAL  ZOOLOGY 

10  (9)  Skin  usually  spiny;  body  a  tube  within  a  tube;  marine.     Star- 

fishes,  sea-urchins,   brittle   stars,   sea-cucumbers,   stone-lilies: 

Phylum  IX,  ECHINODERMATA. 

11  (4)  Bilaterally    symmetrical.     Have    anterior    (front),    posterior 

(hind),   right   and   left,    dorsal    (back),    and   ventral    (belly) 
regions 12 

12  (15)  Body  soft,  flat,  worm-like 13 

13  (14)  One  opening  into  digestive   cavity;  or  no  digestive  cavity. 

Phylum  V,  PLATYHELMIA. 

14  (13)  With   mouth   and   anus Phylum  VI,   NEMERTINA. 

15  (12)  Body  not  flat  and  leaf-like 16 

16  (17)  Body  cylindrical,  worm-like;  without  anterior  ciliated*  lobes. 

Phylum  VII,  NEMATOIDEA. 

17  (16)  Body  not  as  described  under  16 18 

18  (21)  Small,  often  microscopic,  aquatic  animals;  solitary  or  growing 

in  branching  plant-like  colonies 19 

19  (20)  Microscopic  worm-like  animals  with  two  ciliated*  lobes  at  the 

anterior  end  and  a  chewing  stomach  within  the  body. 

Phylum  VIII,  ROTIFERA. 

20  (19)  Attached   colonial   animals   which   have   a   superficial   radial 

symmetry Phylum  X,  BRYOZOA. 

21  (18)  Larger  animals;  often  possessing  a  hard  shell  which  may  cover 

the  body 22 

22  (23)  Stalked  attached  marine  animals  with  a  shell  composed  of  two 

(dorsal  and  ventral)  valves.    Lamp  shells. 

Phylum   IX,   BRACHIOPODA. 

23  (22)  Soft-bodied  animals,  usually  with  a  shell  and  a  ventral  muscular 

"foot" Phylum  XIV,  MOLLUSCA. 

24  (3)  Body  metameric  and  bilaterally  symmetrical 25 

25  (28)  No    jointed    appendages,    body    worm-like    (snakes    belong 

under  30) 26 

26  (27)  Microscopic  animals,  with  ciliated  lobes  at  the  anterior  end  of 

the  body  and  a  chewing  stomach  within. 

Phylum  VIII,  ROTIFERA. 

27  (26)  No  ciliated  lobes  at  anterior  end;  body  with  minute  bristles 

along  the  sides  or  with  an  adhesive  sucker  at  one  or  both  ends. 

Phylum  XII,  ANNELIDA. 

28  (25)  Jointed  appendages   (except  in  limbless  amphibians,  snakes, 

and  limbless  lizards) 29 

29  (30)  External  skeleton  (turtles,  which  have  both  exo-  and  endo- 

skeletons,  belong  under  30) ;  no  chorda. 

Phylum  XIII,  ARTHROPODA. 

30  (29)  Endoskeleton;  chorda  or  backbone  present. 

Phylum  XV,  CHORDATA. 
*  Cilia  are  microscopic  hair-like  processes  which  usually  wave  actively. 


CLASSIFICATION  OF  ANIMALS  21 

CHIEF  PHYLA  OF  THE  ANIMAL  KINGDOM 

Subkingdom  PROTOZOA 

1.  Protozoa  (protos,  first;  zoon,  animal). — Animals  made 
up  of  a  single  cell.     The  cells  may  grow  together  in  colonies 
but  never  show  any  division  of  labor  as  tissues.     Examples: 
Amoeba,  Euglena,  Paramoecium,  malarial  parasite,  Volvox. 
8500  species. 

Subkingdom  METAZOA 

2.  Porifera  (porus,  pore;  fero,  carry). — The  body  radial 
or  without  definite  symmetry,  filled  with  numerous  small 
inhalent  pores,  and  with  one  or  more  large  apertures  where 
water  is  expelled.     Cellular  tissues  and  germ,  or  repro- 
ductive, cells  are  well  developed,  but  not  laid  down  in 
fundamental  germ  layers.     A  skeleton  made  up  of  lime, 
silica,  or  horny  material  is  often  present  throughout  the 
body.     Peculiar  cells  bearing  collars  line  some  of  the  canals 
cf  the  interior.     Tissues  are  well  developed  but  there  are 
no  organs.     This  phylum  includes  the  sponges,  which  are 
attached  and  usually  grow  up  from  the  bottom  somewhat 
like  water  plants.     2500  species. 

3.  Coelenterata  (koilos,  hollow;  enteron,  intestine). — The 
body  radially  symmetrical,  usually  with  four  or  six  anti- 
meres;  diploblastic  (with  two  primary  cell-layers);  with  a 
single  gastrd-vascular  cavity  and  no  anus.     The  body-wall 
is  provided  with  peculiar  stinging  structures — the  nemato- 
cysts.     Examples:  Hydra,   jellyfishes,   zoophytes,   polyps, 
and  corals.     4400  species. 

4.  Ctenophora  (ktenos,  a  comb;  phoreo,  I  bear). — Animals 
possessing  radial  combined  with  bilateral  symmetry;  with 
eight  radially  arranged  rows  of  paddle  plates;  triploblastic 
(with  three  primary  cell  layers) ;  anus  usually  present.     Sea 
Walnuts  or  Comb-jellies.     100  species. 

5.  Platyhelmia  (platys,  flat;  helminthos,  a  worm). — Flat, 
bilaterally  symmetrical,  triploblastic  animals;  with  a  single 
gastro- vascular  cavity  having  no  anus,  or  without  a  diges- 


22  GENERAL  ZOOLOGY 

tive  system;  a  very  small  coelom  present.  A  well-developed 
system  of  excretory  tubes  present.  Flatworms :  planarians, 
flukes,  tapeworms.  5000  species. 

6.  Nemertina. — Flattened   worms   which   resemble   the 
Platyhelmia  in  general  appearance,  but  are  more  special- 
ized.    An  anus  is  present  at  the  posterior  end  of  the  body. 
The  nemerteans.     280  species. 

7.  Nematoidea.     (Nematos,   a  thread). — Cylindrical  or 
thread-like,  bilaterally  symmetrical,  non-met americ,  triplo- 
blastic  worms.     A  mouth  and  anus  are  present  and  the 
body  is  of  the  "  tube-within-a-tube  type."     The  body  cav- 
ity is,   however,  unlined  and  hence  not  a  true  ccelom. 
Round- worms:  Trichina,  hook-worm,  vinegar-" eels,"  etc. 
1500  species. 

8.  Rotifera  (rota,  a  wheel;  /ero,  I  carry). — Microscopic, 
bilaterally  symmetrical,  metameric,  triploblastic  animals; 
with  a  coelom;  two  ciliated  lobes,  which  often  look  like 
rotating  wheels,  at  the  anterior  end.     The  digestive  system 
is  tubular,  with  a  mouth,  anus,  and  several  distinct  regions. 
There  are  chewing  teeth  in  the  stomach.     A  pair  of  water 
canals  are  present  which  serve  for  excretion.     The  wheel- 
animalcules.     500  species. 

9.  Brachiopoda    (brachion,    the   arm;   pous,    a   foot).— 
Marine,    bilaterally    symmetrical,    triploblastic    animals; 
which  possess  a  bivalve  shell  (valves  dorsal  and  ventral) 
and  are  usually  attached  by  a  stalk.     A  peculiar  horseshoe- 
shaped  structure  in  front  of  the  mouth  gives  the  name  to 
this  phylum.     The  body  is  very  small  in  proportion  to  the 
size  of  the  shell.     The  lamp  shells,  which  are  mostly  extinct, 
are  examples.     500  living  species. 

10.  Bryozoa  (6rwm,  moss;  zoon,  animals) . — Small,  usually 
colonial  animals,  which  are  mostly  marine.     The  colony 
usually  has  a  branching  plant-like  form.     At  the  tips  of  the 
branches  are  the  individual  animals  which  have  a  super- 
ficial radial  symmetry.     A  number  of  tentacles  surround 
the  mouth  and  a  U-shaped  digestive  tube  leads  to  the 
anus.     1200  species. 


CLASSIFICATION  OF  ANIMALS  23 

11.  Echinodermata  (echinos,  hedgehog;  derma,  skin).— 
Radially  symmetrical  animals,  usually  with  five  antimeres. 
A  large  coelom  is  present,  and  there  is  a  peculiar  system 
of  tubes  called  the  water-vascular  system.     An  anus  may 
be    lacking    and    is    often    non-functional    when   present. 
There  is  usually  a  calcareous,  spiny  skeleton.     Starfishes, 
brittle-stars,  sea-cucumbers,  sea-urchins,  stone-lilies.     4000 
species. 

12.  Annelida    (annulus,    ring). — Metameric,    bilaterally 
symmetrical,  triploblastic ,  soft-bodied  worms.     Body  clearly 
a  tube  within  a  tube — i.e.,  coelom  very  well  developed ;  paired 
excretory  organs  (nephridia)  in  each  segment  of  the  body; 
no  j  ointed  appendages.    Earthworm,  marine  worms,  leeches. 
4000  species. 

13.  Arthropoda  (arthron,  joint;  pous,  foot). — Metameric, 
bilaterally   symmetrical   animals;   with   a   chitinous   exo- 
skeleton  arid  jointed  appendages;  coelom  poorly  developed. 
Crustaceans,  insects,  spiders,  centipedes,  scorpions,  ticks. 
400,000  species. 

14.  Mollusca  (molis,  soft). — Non-met americ,  bilaterally 
symmetrical,   triploblastic   animals;  in  which  there  is  a 
well-developed  digestive  system  with  mouth  and  anus,  a 
small  coelom,  and  usually  a  calcareous  exoskeleton  in  the 
form  of  a  shell.     Clams,  mussels,  snails,  slugs,  devil-fishes, 
octopi.     60,000  species. 

15.  Chordata    (chorda,   cord   or   string). — Triploblastic, 
metameric,    bilaterally   symmetrical   animals;   with   well- 
developed  coelom  and  an  endoskeleton — the  chorda  or  other 
supporting    structures.     Paired    appendages    are    usually 
present,  and  these  are  often  jointed.     Fishes,  newts,  frogs, 
reptiles,  birds,  mammals.     36,000  species. 

The  relationships  of  the  phyla  are  shown  in  Fig.  18. 


24 


GENERAL  ZOOLOGY 


Mammalia 


Protozoa 
FIG.   18. — Showing  the  probable  relationships  of  the  phyla  of  animals. 


CHAPTER  III 

LIFE  AND  LIVING  THINGS 

"Just  as  the  search  for  the  philosopher's  stone  that  was  to  transmute 
the  baser  metals  into  gold,  led  through  alchemy  to  the  foundations  of 
modern  chemistry,  and  to  a  richer  reward  than  the  long-sought  stone, 
and  as  the  vain  pursuit  of  the  elusive  elixir  vitce,  that  was  to  renew  youth 
and  vigour  and  give  unending  life  at  the  prime,  merged  into  the  begin- 
nings of  scientific  medicine;  so  the  inquiry  into  spontaneous  generation, 
or  the  origin  of  life,  opened  up  the  whole  of  our  modern  knowledge  of  the 
causation  of  disease  through  the  discoveries  of  Pasteur,  and  onward 
beyond  that  laid  the  broad  foundations  for  the  wonderful  developments 
of  modern  surgery  which  arose  from  the  noble  lif ework  of  Lister.  Millions 
of  lives  have  been  saved,  and  untold  misery  and  suffering  averted, 
by  practical  discoveries  which  arose  from  apparently  purely  philosoph- 
ical enquiries  dealing  with  theories  which  might  have  been  dismissed  as 
chimerical."  Moore. 

What  is  life?  No  one  knows!  Definitions  have  been 
formulated  which  will  enable  us  to  separate  living  from 
non-living  things,  but  when  it  comes  to  knowing  why  a 
living  thing  is  alive  we  have  little  that  is  satisfying  from  a 
scientific  point  of  view.  Things  are  alive  because  they 
have  life.  It  may  seem  idle  to  pursue  a  discussion  which 
in  the  end  will  lead  us  back  to  where  we  started,  but  we 
may,  nevertheless,  look  with  profit  into  the  facts  and 
theories  which  relate  to  living  things.  Because  we  cannot 
answer  a  question  is  no  reason  why  we  should  not  try. 
Maybe,  sometime,  by  trying,  the  answer  will  be  found. 

ORIGIN  OF  LIFE 

In  the  past  many  curious  notions  have  been  held  in  regard 
to  the  origin  of  individual  animals.  It  was  at  one  time 
commonly  believed  that  small  aquatic  animals  generated 
from  mud,  and  that  decaying  meat  was  transformed  into 

25 


26  GENERAL  ZOOLOGY 

maggots.  Van  Helmont,  a  deservedly  eminent  scientist  of 
the  sixteenth  century,  was  a  firm  believer  in  such  spon- 
taneous generation,  and  soberly  stated  that  mice  could  be 
produced  by  placing  some  dirty  linen  in  a  receptacle,  to- 
gether with  a  few  grains  of  wheat  or  a  piece  of  cheese. 
Such  crude  ideas  show  the  low  level  of  the  scientific  thought 
of  the  time.  The  belief  that  organisms  commonly  origi- 
nated from  inorganic  materials  or  decaying  substances  was 
prevalent. 

The  first  critical  experiments  which  helped  to  do  away 
with  the  belief  in  spontaneous  generation  were  performed 
by  an  Italian  poet  and  physician,  Franchesca  Redi.  He 
demonstrated  clearly  that  fly  maggots  were  not  engendered 
spontaneously  in  spoiled  meat,  but  developed  from  eggs 
deposited  by  flies.  He  set  three  jars  on  his  window  sill- 
one  was  uncovered;  one  covered  with  gauze  so  that  the  air 
might  enter  freely  but  no  flies  could  come  in;  the  other  was 
corked  tightly.  Though  the  meat  in  all  the  bottles  spoiled, 
maggots  appeared  only  in  the  uncovered  jar.  The  flies 
were  seen  to  lay  their  eggs,  and  the  hatching  of  maggots 
was  observed.  Redi's  epoch-making  discovery  seems  very 
simple  to  us,  but  it  aroused  a  storm  of  discussion  and  dis- 
pute in  his  time. 

It  would  perhaps  be  expected  that  the  invention  of  the 
microscope  would  at  once  have  furnished  the  means  to  dis- 
prove spontaneous  generation,  but  it  had  the  opposite 
effect.  This  instrument  opened  up  a  new  world  of  minute 
"  animalculae,"  and  with  these  the  advocates  of  spon- 
taneous generation  made  their  last  stand.  It  was  found 
that  if  dry  hay  was  put  in  water,  cooked  to  destroy  any 
living  germs  which  might  be  present,  and  allowed  to  stand 
for  a  day  or  two,  that  the  " infusion"  was  filled  with  myriads 
of  "  animalcules."  This  experiment  was  believed  to  prove 
that  the  organisms  were  generated  from  the  disintegrating 
hay.  Later,  however,  it  was  urged  that  the  animalcules 
might  enter  the  infusion  from  the  air.  Experiments  were 
tried  in  which  infusions  were  sealed  up  in  tubes,  both  before 


LIFE  AND  LIVING  THINGS  27 

and  after  boiling.  Spallanzani,  in  the  middle  of  the 
eighteenth  century,  wrote:  "I  used  hermetically  sealed 
vessels.  I  kept  them  for  an  hour  in  boiling  water,  and 
after  opening  and  examining  their  contents  after  a  reason- 
able interval,  I  found  not  the  slightest  trace  of  animalcule, 
though  I  examined  with  the  microscope  the  infusions  from 
nineteen  different  vessels."  The  question  was  still  dis- 
cussed, however,  and  even  as  late  as  1858  a  French  scientist 
read  a  paper  before  the  National  Academy  in  which  he 
announced  that  animalcule  had  been  raised  in  boiled  in- 
fusions exposed  only  to  "  artificial  air,"  or  oxygen.  He 
maintained  that  the  organisms  could  not  have  arrived  as 
air-borne  particles. 

This  paper  excited  the  interest  of  Pasteur  and  led  to  the 
work  which  finally  laid  the  idea  of  spontaneous  generation 
to  rest.  "He  showe'd  that  sterilized  cultures  always  be- 
came infected  when  exposed  to  air;  that  properly  filtered  or 
sterilized  air  never  caused  infection;  that  Alpine  air  almost 
free  from  germs  scarcely  ever  produced  a  growth  of  organ- 
isms; that  city  air  nearly  always  produced  contamination; 
and  that,  in  absence  of  added  germs  from  without,  culture 
media  remained  sterile  for  years.  The  sources  of  error  in 
the  work  of  his  opponents  were  elucidated,  and  their  con- 
trary results  explained  on  such  grounds."  Tyndall  alfe'# 
furnished  evidence  of  a  physical  nature  which  corroborated, 
the  discoveries  of  Pasteur.  He  was  trying  to  get  a  beam  of 
light  which  was  perfectly  free  from  dust  particles  and  made 
an  apparatus  in  which  the  light  was  passed  through  win- 
dows in  an  air-tight  box.  When  the  box  was  absolutely 
dust-free  there  was  no  fermentation  within  it,  thus  demon- 
strating that  the  origin  of  bacteria  and  similar  organisms 
was  due  to  air-borne  particles. 

At  the  present  time,  the  only  generalization  we  can  make 
concerning  the  origin  of  living  things  is,  "all  life  comes  from 
life."  Every  living  organism  originates  from  a  preexisting 
individual  of  its  own  sort.  This  conclusion  is  unsatisfying, 


28  GENERAL  ZOOLOGY 

from  a  scientific  point  of  view,  and,  of  course,  does  not  ex- 
plain the  origin  of  life. 

PHYSICAL  BASIS  OF  LIFE 

If  we  cannot  explain  the  origin  of  life  we  may  at  least  look 
into  the  nature  of  living  matter.  The  name,  protoplasm, 
has  been  given  to  the  complex  mixture  of  chemical  com- 
pounds which  makes  up  the  living  substance.  It  is  a  trans- 
parent semi-fluid  material,  somewhat  like  the  white  of  an 
egg.  At  one  time  there  was  considerable  dispute  in  regard 
to  the  structure  of  protoplasm.  One  school  maintained 
that  there  were  granules  which  were  all-important;  others, 
that  little  fibers  were  the  living  elements;  a  third  group 
that  little  bubbles  were  the  necessary  feature.  Modern 
investigation  has  shown,  however,  that  none  of  these  things 
are  of  prime  importance.  The  same  bit  of  protoplasm  may 
at  different  times  be  granular,  or  reticular,  or  alveolar.  It 
is  more  or  less  clear  and  jelly-like;  it  shows  spontaneous 
internal  movements;  it  is  alive — these  are  the  only  constant 
features  to  be  seen  even  through  the  best  microscopes. 

Protoplasm  is  somewhat  variable  in  its  composition  be- 
cause it  is  unstable  chemically,  and  is  continually  chang- 
ing. Nevertheless,  it  shows  a  general  similarity  of  struc- 
ture and  elemental  composition  in  all  plants  and  animals. 
It  is  made  up  of  twelve  of  the  eighty-odd  known  chemical 
elements.  These  are : 

Carbon,  oxygen,  hydrogen,  nitrogen,  sulphur — 99  percent. 

Chlorine,  phosphorus,  potassium,  sodium,  magnesium, 
calcium,  iron — 1  per  cent. 

Protoplasm  shows  some  chemical  peculiarities  which  are 
noteworthy.  (1)  The  element  carbon  has  remarkable 
ability  to  form  extremely  complex  systems  of  combinations 
with  other  elements  and  its  compounds  are  most  important 
(Fig.  19).  (2)  Protoplasm  is  very  labile,  or  unstable. 
Living  substance  may  readily  incorporate  a  little  more 
water  (or  other  compound)  or  lose  a  little  as  the  necessity 


LIFE  AND  LIVING  THINGS  29 

arises.  This  ability  to  change  continually  yet  be  essentially 
the  same  is  perhaps  the  most  striking  quality  of  protoplasm. 
(3)  The  most  important  and  characteristic  chemical  com- 
pounds in  protoplasm  are  colloidal  —  i.e.j  they  consist  of 
enormous  molecules,  which  mix  readily  with  water  but 
never  completely  dissolve,  as  many  crystalline  substances 
do.  The  most  important  colloidal  substances  are  the 
proteins.  These  contain  carbon,  oxygen,  hydrogen,  and 
nitrogen  and  possess  various  unique  properties.  They  are 
made  up  of  enormous  molecules  which  may  be  readily 


H          S    W,  CHa 


/VH—C—C—C—C—COOH  H—C     -     C—COOH. 

C      H    H  H    H  CtHs    H 

NH* 

Jso/euc/ne 
Arginine 


CH3    /V/i£  A/H2  H    H    H 

H—CCKC—COOH.  H-k—C—C—C—  C—COOH 

CH3    H  H     H     H     H      H 


Leucine  Lys/ne 

FIG.  19.  —  Some  of  the  chemical  compounds  formed  when  protoplasm  breaks 
down.  In  these  diagrams  the  significance  of  the  letters  is  as  follows:  c,  carbon; 
h,  hydrogen;  n,  nitrogen;  o,  oxygen. 

modified  by  adding  or  eliminating  whole  groups  of  atoms. 
If  we  compare  an  atom  to  a  soldier,  we  may  liken  a  protein 
molecule  to  an  army  corps  in  which  a  regiment  of  infantry, 
or  a  hospital  unit,  or  some  other  part  is  continually  being 
transferred  from  one  position  to  another.  One  like  the 
other  has  a  certain  recognizable  unity  —  yet  the  internal 
arrangements  are  never  continuously  the  same. 

The  chemical  compounds  which  make  up  protoplasm  are 
not  all  organic.  Inorganic  compounds  are  present  in  con- 
siderable quantity.  Water  is  an  universal  constituent  of 
protoplasm  and  there  are  always  mineral  salts,  such  as  the 


30  GENERAL  ZOOLOGY 

phosphates,  chlorides,  and  carbonates  of  calcium,  potas- 
sium, magnesium,  and  iron.  Some  jelly  fishes  are  more  than 
99  per  cent,  water. 

Organic  compounds  were  formerly  believed  to  come  only 
from  living  things — but  now  the  chemist  may  make  thou- 
sands of  them  in  his  laboratory,  and  many  are  of  vast 
commercial  importance.  The  chief  classes  of  organic  com- 
pounds are  proteins,  carbohydrates,  and  fats.  Proteins 
differ  from  the  others  in  that  they  contain  nitrogen  in 
addition  to  carbon,  oxygen,  and  hydrogen.  They  consti- 
tute the  greater,  and  most  essential,  part  of  the  protoplasm. 
Lean  meat  and  white  of  egg  are  examples  of  substances 
which  are  largely  made  up  of  proteins.  Carbohydrates,  of 
which  sugars  and  starches  are  examples,  have  their  oxygen 
and  hydrogen  in  the  same  proportions  (H2O)  in  which  they 
occur  in  water.  Fats,  or  hydrocarbons,  contain  the  same 
elements  as  carbohydrates  but  in  different  proportions,  so 
that  there  is  a  smaller  percentage  of  oxygen. 

It  is  possible  for  the  chemist  to  analyze  protoplasm  to 
some  extent  and  to  learn  the  exact  formula?  for  some  of  the 
chemical  compounds  present,  but  its  protein  molecules  are 
so  large  that  their  exact  composition  and  arrnagement  is 
still  a  matter  of  uncertainty  in  most  cases.  Here  again, 
then,  we  are  met  with  an  unsatisfactory  termination  in  our 
search  for  the  fundaments  of  life.  When  we  try  to  analyze 
living  substance,  it  dies,  and  we  have  left  a  number  of  com- 
plex compounds  (Fig.  19)  which,  though  perhaps  simpler 
than  those  present  during  life,  are  still  of  such  complicated 
structure  that  they  are  imperfectly  known. 

MECHANISM  AND  VITALISM 

Notwithstanding  our  comparative  ignorance  concerning 
the  phenomena  of  life  men  have  not  been  backward  in 
putting  forth  theories  to  explain  the  operation  of  living 
things.  There  has  been  long  and  bitter  controversy  be- 
tween the  mechanists,  who  maintain  that  living  matter  is 


LIFE  AND  LIVING  THINGS  31 

not  essentially  different  from  non-living,  and  the  vitalists, 
who  hold  that  living  things  possess  a  vital  principle  which 
makes  them  different  from  non-living. 

The  mechanist  believes  it  is  possible  that  no  living  ma- 
chine is  beyond  experimental  analysis.  He  expects  as  his 
knowledge  grows  broader  to  know  more  and  more  about  the 
mechanism  of  life,  and  even  hopes,  sometime,  to  be  able  to 
make  a  living  organism  himself.  From  a  scientific  point 
of  view  it  is  certainly  not  satisfying  to  believe  that  all 
things  were  created  as  they  are;  that  living  things  are  alive 
because  they  are  alive,  and  that  it  is  of  no  avail  to  try  to 
discover  how  they  came  to  live  and  are  living.  It  would  be 
equally  unscientific  to  say  that  living  things  were  not 
created.  There  is  no  proof  either  way.  Yet  the  mechanist 
has  much  on  his  side. 

The  first  big  victory  for  the  mechanists  was  the  dis- 
covery that  organic  compounds  can  be  made  synthetically. 
When  Woeler  in  1828  made  urea  in  his  laboratory,  he 
opened  up  a  new  field  which  has  been  very  fruitful.  It  had 
been  believed  that  such  substances  could  only  be  elabor- 
ated within  the  bodies  of  living  organisms.  For  example, 
Liebig,  who  was  a  man  of  good  standing  scientifically, 
manufactured  a  beef  extract  which  he  claimed  was  nourish- 
ing because  it  possessed  something  "vital."  One  of  his 
opponents  tested  the  extract,  to  see  if  such  was  the  case. 
He  took  a  litter  of  kittens  from  their  mother.  Half  of 
them  were  fed  nothing  and  the  others  received  beef  extract. 
The  starved  lot  lived  longer  than  the  others,  showing  that 
the  extract  had  little  food  value,  but  was  stimulating. 
When  it  became  known  that  not  only  urea  but  many  other 
organic  compounds  could  be  made  synthetically,  people 
began  to  doubt  the  "  vital  principle." 

There  is  no  question  that  every  organism  is  a  machine. 
It  is  composed  of  matter  which  may  be  separated  into  well- 
known  chemical  elements  and  compounds.  It  consists 
of  levers,  pulleys,  and  other  things  which  work  according 
to  the  laws  of  mechanics.  Many  complicated  activities 


3 


32  GENERAL  ZOOLOGY 

which  were  once  believed  to  show  the  presence  of  some 
vital  quality  are  now  understood  and  may  be  controlled 
at  the  will  of  the  experimenter.  A  turtle's  heart  may  be 
taken  from  its  body  and  placed  in  saline  solution  where  it 
will  continue  to  beat  rhythmically  for  two  or  three  days. 
Furthermore,  its  rate  of  beat  may  be  regulated.  If  a  cer- 
tain mineral  salt  is  added  to  the  solution  in  which  it  lies, 
it  beats  faster;  if  another  is  added,  it  slows  down  and  even 
ceases  to  pulsate,  but  will  begin  again  if  the  solution  is 
restored  to  its  original  condition. 

Another  stroke  for  mechanism  came  with  the  knowledge 
t  of  the  control  of  "  vital"  activities  through  enzymes  and 
^4jormones.  For  example,  there  are  in  every  man  a  pair 
?  of  glands,  the  thyroids,  on  either  side  of  the  neck  which 
\  are  very  necessary  to  "vital"  activity.  They  give  off  a 
substance,  thyroidin,  which  enters  the  blood  stream  and  is 
constantly  necessary  for  the  proper  nourishment  of  the 
body.  Goitre  is  the  name  given  to  a  diseased,  swollen 
condition  which  may  interfere  with  the  proper  function- 
ing of  the  thyroids.  It  was  in  trying  to  find  a  cure  for 
this  trouble  that  some  interesting  facts  were  discovered. 
It  was  found  that,  if  diseased  glands  were  removed,  the 
body  would  waste  away  and  the  patient  finally  died.  But 
if,  after  taking  out  the  glands,  thyroidin  from  a  healthy 
sheep  was  injected  hypodermically,  there  was  no  dis- 
turbance. Furthermore  the  same  result  could  be  obtained 
by  grafting  a  fresh  thyroid  beneath  the  skin.  Thyroidin 
is,  then,  necessary  in  the  body  of  man,  but  it  makes  little 
difference  how  it  is  supplied.  The  machine  needs  a  certain 
amount  of  the  substance  to  run  properly  but  the  thyroid 
gland  is  not  necessary  any  more  than  an  oil  cup  is  neces- 
sary for  an  engine  if  oil  is  supplied  in  some  other  way. 
Many  other  substances  control  or  facilitate  various  activi- 
ties which  were  once  believed  to  be  dominated  by  "  vital 
spirits." 

Arguments  might  be  multiplied  which  tend  to  show  that 
living  animals  are  machines,  but  let  us  look  at  the  other 


LIFE  AND  LIVING  THINGS  33 

side.  The  vitalist  has  only  one  argument — no  mechanist  s 
has  ever  made  a  living  thing  or  brought  a  dead  thing  to  * 
•life.  He  believes  that  even  though  we  could  understand 
living  mechanism  and  its  workings  completely,  we  would 
still  not  be  able  to  produce  life  without  some  preexisting 
life  to  start  it  off.  Vitalism  involves  more  than  this, 
there  is  belief  in  a  fundamental  difference  between  the 
living  and  non-living.  There  must  be  something  ' 'vital'7 
present  without  which  an  organism  will  cease  to  be  alive. 
Some  vitalists  call  this  vital  principle  the  soul.  There  are 
those  who  believe  that  all  living  things  have  souls,  and 
others  who  feel  that  such  a  quality  is  confined  to  a  few 
of  the  higher  animals  or  to  man  alone. 

Until  we  learn  more,  there  can  be  no  final  decision  be- 
tween vitalism  and  mechanism.  The  radical  mechanist 
can  continue  to  say  that  we  have  no  proof  of  anything  vital, 
or  divine,  and  can  assert,  if  he  chooses,  that  "vitality" 
is  simply  a  name  for  a  lot  of  things  we  do  not  yet  under- 
stand. The  vitalist  can  still  hold  that  the  mechanist  has 
never  proved  his  point — living  things  are  different  from 
non-living  and,  through  scientific  reasoning,  we  cannot 
show  why.  A  scientific  attitude  compels  us  to  accept 
nothing  until  it  is  proved.  Very  little  is  yet  proved.  If 
we  knew  more  we  probably  would  not  argue  so  much. 
Professor  Wilson  in  a  recent  address  said,  "And  so,  if 
you  ask  whether  I  look  to  a  day  when  we  shall  know  the 
whole  truth  in  regard  to  organic  mechanism  and  organic 
evolution,  I  answer:  No!  but  let  us  go  forward." 

CHARACTERISTICS  OF  LIVING  THINGS 

It  is  difficult  to  formulate  absolute  differences  which  will 
always  separate  living  from  non-living  objects.  It  is 
possible,  however,  with  some  confidence  to  set  forth  the 
following  six  qualities  which  will  serve  to  characterize  living 
organisms. 

1.  Determine  Size  and  Structure. — All  animals  and  plants 
vary  to  a  considerable  extent,  in  fact,  it  is  often  said  that 


34 


GENERAL  ZOOLOGY 


no  two  are  alike,  yet  each  species  when  adult  has  a  certain 
average  size  and  form  which  enables  one  to  recognize  it. 
A  cat  is  never  as  large  as  a  cow;  a  birch  tree  is  never  found 
with  the  bark  or  leaves  of  an  oak.  Living  things  cling  to 
such  specificities  generation  after  generation. 

2.  Precise  Elementary  Composition. — Living  matter, 
though  highly  variable,  is  made  up  of  only  twelve  of  the 
eighty  odd  chemical  elements  available  in  nature.  These 


Excretions 


Oxtjqen 


Oxidation 
O\erqy 


Rxeccs     food  material 

FIG.  20. — A  scheme  showing  metabolism,  or  the  chemical  changes  continu- 
ally going  on  in  protoplasm.  No  living  thing  can  stand  still.  It  must  change 
continually. 

occur  in  definite  proportions  and  form  only  certain  kinds 
of  compounds. 

3.  Definite  Organization. — Living  organisms  are  composed 
of  a  variety  of  unlike  parts  which  are  generally  incapable 
of  independent  existence.  The  smallest  unit  of  organic 
structure  is  a  cell  and  in  large  animals  there  may  be  billions. 
The  parts  of  an  animal  will  not  exist  separately — you  can- 
not remove  a  kidney  or  a  stomach  and  have  it  remain 


LIFE  AND  LIVING  THINGS  35 

active  because  the  functioning  of  such  an  organ  depends 
upon  its  surroundings. 

4.  Metabolism  is  the  name  for  the  complex  of  chemical 
changes  continually  taking  place  in  living  things.  Noth- 
ing which  is  alive  can  stand  still,  it  must  change.  Some 
changes  are  constructive  (anabolism),  some  are  destructive 
(catabolism).  In  general  there  is  an  orderly  sequence  in 
the  transformation  of  substance.  Material  is  appro- 
priated, digested  (changed  chemically  and  dissolved),  and 
assimilated  (or  converted  into  living  substance).  If  any 
of  the  material  is  not  fit  for  food,  it  is  rejected  as  fceces. 
In  order  to  furnish  energy  to  transform  materials  and 
build  substance,  oxidation  takes  place,  and  this  makes 


FIG.  21. — Scheme  showing  the  life  cycle  through  which  every  living  thing  must 

pass. 

respiration  necessary  to  supply  the  oxygen.  After  living 
substance  has  been  broken  down  a  residue  is  left  which 
passes  out  as  an  excretion  (Fig.  20). 

In  addition  to  this  continual  daily  rhythm  there  is 
another  series  of  periodic  changes  which  is  characteristic 
of  living  things.  This  is  called  the  Life  Cycle,  during  which 
an  organism  passes  through  the  stages  of  youth,  maturity, 
and  old  age.  A  young  animal  has  great  vitality — it  grows 
rapidly  and  has  an  abundance  of  energy.  A  mature  animal 
is  at  its  best — the  machine  is  complete  and  perfect.  Usu- 
ally at  this  stage  reproduction  takes  place  and  a  new 
cycle  is  started  by  setting  free  a  young  animal.  In  old 
age  the  machine  wears  out  and  death  finally  puts  an  end 


36  GENERAL  ZOOLOGY 

to  the  cycle.     Every  animal  must  run  in  the  race  of  life; 
and  the  goal  for  all  is  the  same — death.* 

5.  Reproduction. — All  living  things  have  the  power  to 
perpetuate  their  race.     Some  animals  split  themselves  in 
two,  or  break  up  into  fragments,  and  each  of  the  pieces 
grows  into  a  complete  new  individual.     Such  multiplica- 
tion increases  the  number  of  organisms  easily  and  quickly. 
At  times,  however,  most  animals  have  some  means  of  proper 
reproduction   during   which   there   is   fusion   of    products 
from  two  individuals  to  form  a  third — and  this  new  indi- 
vidual starts  out  with  all  the  vigor  of  youth,  though  its 
parents  may  have  been  at  the  other  end  of  the  cycle. 

6.  Adaptability  or  Reactiveness. — All  organisms  are  able 
to  adjust  themselves  to  their  surroundings  and  this  ability 
to  meet  changing  conditions  is  perhaps  the  most  character- 
istic  attribute  of  living  things.     Garden  spiders  always 
spin  webs  of  the  same  type,  yet  one  may  fill  a  square,  and 
another  a  triangular  space.     Individuality  is  thus  retained 
but  adapted  to  varying  conditions.     The  success  of  any  man 
depends  upon  his  adaptability — how  well  he  can  meet  the 
difficulties  and  changing  conditions  of  life. 

*  Except  in  some  Protozoa,  which  may  escape  death  from  old  age  by 
becoming  young  again  (see  Chapter  XII). 


CHAPTER  IV 
CELLS 

Protoplasm,  or  living  substance,  occurs  only  in  the  form 
of  cells.  These  " units  of  life"  are  in  a  certain  sense  like 
the  bricks  from  which  men  make  houses.  Bricks  are  of 
various  colors  and  shapes,  they  may  be  built  into  houses 
or  they  may  exist  as  separate  chunks  of  burnt  clay,  yet 
anyone  recognizes  a  brick  when  he  sees  it.  Brick  houses 
may  have  doors,  various  arrangements  of  windows,  porches, 
and  other  embellishments  without  changing  the  essential 
character  of  the  bricks  which  furnish  the  bulk  of  their 
mass. 


FIG.  22. — Cells  scraped  from  the  inside  of  a  man's  cheek. 

Living  substance  always  occurs  in  the  form  of  cells. 
Many  of  these  " units  of  life"  are  independent  (Protozoa); 
they  are  able  to  reproduce  themselves,  carry  on  metabolism, 
and  maintain  all  the  other  activities  characteristic  of  living 
things,  as  described  in  Chapter  III.  Other  cells  live  as 
small  units  in  the  midst  of  great  living  machines  and  are 
incapable  of  independent  existence.  A  recent  estimate 
places  the  number  of  the  nerve  cells  in  the  cortex  of 
the  human  brain  at  9,280,000,000,  and  yet  these  would 
together  occupy  only  about  one  cubic  inch  of  space.  The 

37 


38 


GENERAL  ZOOLOGY 


greater  part  of  the  brain  substance  is  made  up  of  nerve 
fibers,  other  types  of  cells,  and-  their  products.  The  whole 
body  contains  a  vast  number  of  such  units.  Though  cells 
are  highly  variable  in  size  and  shape,  they  have  certain 
structures  which  are  familiar  to  everyone  who  has  looked 
through  a  microscope  at  any  thing  living. 

If  you  scrape  a  little  of  the  membrane  from  the  inside  of 
your  cheek  and  place  it  under  a  microscope,  the  fragments 
will  appear  like  Fig.  22.  The  individual  cells  are  flattened 
and  irregularly  hexagonal  in  form.  Each  one  has,  at  the 
center,  a  body  of  somewhat  denser  material,  the  nucleus. 


FIG.  23. — Examples  of  cells.  A,  cartilage;  c,  cells;  s,  the  substance  between 
the  cells.  B,  a  muscle  cell  from  a  round- worm;  m,  muscular  or  contractile 
portion;  n,  nucleus.  C,  nerve  cell;  a,  axis  cylinder  process;  d,  dendrites,  or  root- 
like  projections;  n,  nucleus. 

The  rest  of  the  cell  substance,  i.e.,  the  outside  part  which 
surrounds  the  nucleus,  is  the  cytoplasm.  Other  parts  of 
the  body  will  show  cells  somewhat  like  those  from  the 
cheek.  All  have  a  nucleus  and  cytoplasm,  but  there  may 
be  great  variation  in  both.  Some  cells  have  walls  of 
secreted  material  around  them  which  forms  "  cell-walls," 
others  have  the  cytoplasm  drawn  out  into  long  projections, 
or  vary  in  other  ways.  Fig.  23  shows  a  few  examples  of 
cells. 

Fig.  24  gives  a  general  scheme  of  a  typical  cell  with  all 


CELLS 


39 


the  structures  commonly  present.  The  cytoplasm  is  usu- 
ally made  up  of  fibers,  granules,  and  fluid  plasma.  Other 
bodies  such  as  crystals,  oil  globules,  and  water  vacuoles, 
may  be  present.  Such  " extra"  bodies  are  called  plastids, 
metaplasm,  etc.  The  nucleus  is  covered  by  a  membrane 
and  has,  besides  some  fluid  plasma,  a  network  of  linin 
fibers  to  which  are  attached  irregular  masses  of  chromatin. 
Just  outside  the  nucleus  is  the  centrosphere  which  contains 
two  centrosomes. 


CELL 

MEMBRANE: 


NUCLEAR  _  i 
MEMBRANE. 


PLASTIDS'-— 


CYTOPLASMIC 
'   NETWORK 


FIG.  24. — A  typical  cell  showing  all  the  parts. 

There  is  a  division  of  labor  among  the  structures  men- 
tioned. The  plasma  membrane  on  the  outside  of  a  cell 
is  semi-permeable,  allowing  certain  substances  to  pass  into 
or  out  of  the  cytoplasm  but  not  admitting  others.  The 
cytoplasm  does  the  work  which  is  the  particular  task  of 
each  cell.  In  a  muscle  cell  it  is  specialized  for  contraction; 
in  a  nerve  cell  it  carries  the  nervous  impulses.  The  cyto- 
plasm may  also  store  food  or  even  waste  products.  The 
general  function  of  the  nucleus  appears  to  be  the  control 
of  synthetic  metabolism.  A  cell  without  a  nucleus  may 
work  until  its  reserve  energy  is  exhausted,  but  it  can  never 
build  up  any  new  material.  The  chromatin  in  the  nucleus 


40  GENERAL  ZOOLOGY 

appears  to  be  the  chief  bearer  of  hereditary  qualities  from 
one  cell  to  another.  How  the  transfer  of  hereditary  char- 
acters is  believed  to  take  place  will  be  explained  later 
(Chapter  XXX).  The  centrosomes  are  apparently  inac- 
tive except  when  the  cell  divides.  They  then  commonly 
form  dynamic  centers  which  are  concerned  with  the 
separation  of  the  chromatin  material. 

Cells  rarely  exceed  the  size  which  is  usual  for  their  par- 
ticular kind.  The  largest  in  animals,  such  as  some  nerve 
cells,  are  two  or  three  feet  long.  Most  cells,  however,  are 
much  smaller  and  usually  cannot  be  seen  with  the  naked 
eye.  The  smallest  bacteria  are  less  then  one  twenty-five 
thousandth  of  an  inch  in  diameter.  When  a  cell  has  grown 
so  that  it  threatens  to  exceed  the  maximum  set  for  its  kind 
by  natural  causes,  it  divides.  Cell-division  is  important 
in  several  ways.  It  allows  cells  to  grow  without  becoming 
too  large,  and  it  increases  the  number  of  cells.  In  many 
celled  animals,  growth  is  closely  bound  up  with  cell- 
division  and  cell-differentiation.  Cell-division  is,  there- 
fore, a  very  important  biological  phenomenon.  There  are 
two  types:  mitotic  and  amitotic. 

MITOSIS  OR  INDIRECT  CELL-DIVISION 

Mitotic  cell-division  is  a  rather  complicated  process 
which  results  in  the  accurate  division  of  the  chromatin  of 
the  nucleus  and  a  somewhat  less  precise  separation  of  the 
other  parts  of  the  cell.  The  changes  which  take  place  are 
grouped  under  five  stages  (shown  in  Fig.  25)  which  may  be 
described  as  follows: 

1.  Resting  Cell. — A  resting  cell  ready  for  division  does 
not  differ  from  any  other  cell,  except  that  the  nucleus  is 
large  in  proportion  to  the  cytoplasm.     The  centrosomes 
lie  at  one  side  of  the  nucleus  and  the  chromatin  is  scattered 
irregularly  through  the  linin  network. 

2.  Prophase. — The   centrosomes   move   apart   and   take 
positions  on  opposite  sides  of  the  nucleus.     Long  strands 


CELLS 


41 


stretch  out  from  them  in  all  directions  and  in  the  zone 
between  the  centrosomes  these  form  a  framework,  the 
spindle.  The  chromatin  has  meantime  formed  a  long 
thread,  the  spireme,  and  this  later  breaks  up  into  a  number 
of  compact  rods  of  chromatin  material,  the  chromosomes. 
The  nuclear  membrane  now  breaks  down  and  the  chromo- 
somes arrange  themselves  around  the  center  of  the  spindle 
halfway  between  the  centrosomes. 

3.  Metaphase. — Each  chromosome  splits  into  two  equal 
parts. 


>ndle 


Spin 


chromosomes 


Prophose 


F 
Mefophase  A/iapha^e  Te/ophase 

FIG.  25. — Mitosis,  or  indirect  cell  division.     The  ehromatiii  is  very  accurately 
divided  because  it  is  the  bearer  of  hereditary  qualities. 

4.  Anaphase. — The  new  chromosomes  formed  by  split- 
ting in  the  last  stage  migrate  toward  the  nearest  centro- 
some.     Thus  half  go  toward  each  pole  of  the  cell. 

5.  Telophase. — The  chromosomes  break  up  into  scattered 
bits  of  chromatin  and  a  nuclear  membrane  is  formed  about 
each  of  the  two  groups.     The  centrosomes  divide  and  the 
new  pairs  come  to  rest  beside  their  respective  nuclei.     A 
separation  takes  place  in  the  cytoplasm  between  the  two 
nuclei,  thus  completing  mitosis  and  forming  two  cells. 

It  seems  strange  that  the  division  of  a  minute  bit  of  liv- 
ing matter  should  involve  such  a  complicated  mechanism 


42  GENERAL  ZOOLOGY 

as  has  been  described.  The  purpose  of  mitosis  is  appar- 
ently to  secure  the  accurate  division  of  the  chromosomes— 
so  that  each  daughter  cell  may  receive  exactly  the  same 
amount  of  chromatin.  The  chromosomes  are  believed  to 
be  the  chief  bearers  of  hereditary  qualities  and  have  been 
known  as  the  " vehicles  of  inheritance."  This  perhaps 
explains  why  such  care  is  taken  to  insure  accurate  division. 
There  is  a  characteristic  number  of  chromosomes  for 
each  plant  or  animal  and  when  one  of  its  cells  divides  the 
same  number  is  always  reformed  from  the  scattered  chro- 
matin of  the  nucleus.  Thus  the  number  in  several  animals 
is  as  follows: 

Ascaris,  a  nematode 2  or  4     Rana,  a  frog;  and  man 24 

Aphis,  a  plant  louse 8     Lumbricus,  an  earthworm 3% 

Gryllotalpa,  a  cricket 12     Crepidula,  a  snail 60 

Rat 16     Artemia,  a  brine-shrimp 168 

It  is  known  that  species  in  the  same  genus  show  constant 
differences  in  the  number  of  chromosomes.  Each  shows  its 
characteristic  number  generation  after  generation.  Some- 
times there  is  an  odd  chromosome.  This  peculiarity  is 
particularly  characteristic  of  the  sex  cells  of  certain  animals. 
In  such  cases,  the  minute  male  sex  cell,  or  spermatozoon, 
may  carry  the  odd  chromosome  and  is  believed  to  produce 
a  female  when  it  "fertilizes"  an  egg;  one  which  has  none 
will  give  rise  to  a  male.  The  odd  chromosome,  therefore, 
perhaps  serves  to  determine  sex. 

AMITOSIS  OR  INDIRECT  CELL-DIVISION 

When  the  cell  divides  without  mitosis  the  process  is  very 
simple.  The  nucleus  elongates,  becomes  constricted  in  the 
middle  like  a  dumbbell,  and  finally  is  pinched  in  two  (Fig. 
26).  The  two  daughter  nuclei  may  differ  greatly  in  size 
and. in  the  amount  of  chromatin  they  contain.  Amitosis 
often  takes  place  in  old,  weak,  or  degenerate  cells  and  may 
be  looked  upon,  at  least  in  some  cases,  as  a  sign  of  a  general 
wearing  out  of  the  cell  mechanism.  Some  students  of  cell 
physiology  dispute  this  interpretation,  however. 


CELLS  43 

Amitotic  cell-division  is  often  a  means  of  increasing  the 
nuclear  surface  in  a  cell.  In  many  active  cells  the  nucleus 
becomes  very  much  branched,  and  it  is  only  a  step  beyond 
this  to  have  it  break  up  into  pieces.  It  will  be  remembered 
that  the  nucleus  controls  synthetic  metabolism  in  the  cyto- 
plasm around  it.  The  cell  can  build  up  faster  with  the  in- 
creased surface  brought  about  by  division,  just  as  man  can 
freeze  ice  cream  faster  with  small  pieces  of  ice  than  he  can 
with  large  chunks. 

Cell-division  does  not  take  long — in  some  cases  from  ten 
minutes  to  half  an  hour.  Many  cells  in  your  body  have 
divided  mitotically  since  you  started  to  read  this  chapter. 
As  to  differences  between  the  two  types — mitosis  is  pri- 


Fio.  26. — Amitosis,  or  direct  cell  division.  The  nucleus  elongates  and  pinches 
in  two.  A  membrane  is  formed  between  the  two  new  nuclei,  thus  making  two 
cells.  Amitosis  is  a  quick  method  of  increasing  nuclear  surface. 

marily  for  the  exact  division  of  the  chromatin;  amitosis  is 
chiefly  for  increasing  the  amount  of  nuclear  surface  in  pro- 
portion to  the  cytoplasm. 

THE  CELL  THEORY 

The  modern  "cell  theory"  asserts  that  cells  are  the  units 
which  build  every  living  organism.  No  plant  or  animal 
exists  which  does  not  have  the  protoplasm  divided  upon 
into  cells.  Though  this  view  is  generally  accepted  by  the 
scientific  world,  it  is  less  than  a  hundred  years  since  it  was 
pronounced. 

In  1665  an  Englishman  named  Hooke,  when  making  a 
careful  examination  of  a  piece  of  bark,  discovered  that  it 
was  made  up  of  little  compartments  arranged  in  regular 


44  GENERAL  ZOOLOGY 

rows  (Fig.  12).  Likening  these  to  the  rooms  in  monasteries 
Hooke  gave  them  the  name,  cells. 

Later  workers  found  cells  in  various  plants  and  animals, 
but  they  were  merely  looked  upon  as. interesting  features 
without  particular  importance.  It  was  in  1833  that 
another  Englishman,  Brown,  discovered  the  nucleus,  which 
he  described  in  certain  plant  cells.  Not  until  1838-1839 
was  the  Schleiden-Schwann  Cell  Theory  brought  before  the 
scientific  world  by  the  two  Germans  whose  name  it  bears. 
In  1838  Schleiden  asserted  that  all  plants  were  made  up 
of  cells;  and  in  the  next  year,  Schwann  published  a  great 
work  which  convinced  everyone  that  not  only  plants  but 
animals  as  well  were  made  up  wholly  of  cells. 

No  generalization,  unless  it  be  Darwin's  "  Origin  of 
Species, "  has  so  stimulated  biological  investigation  as  the 
cell  theory.  A  new  field  was  opened  up  which  led  to  the 
discovery  of  the  method  of  the  fertilization  of  eggs  and  other 
fundamental  matters  which  had  not  been  previously 
understood. 

It  is  unfortunate  that  the  name  "cell"  was  given  to  pro- 
toplasmic units.  One  thinks  of  a  cell  as  a  box  or  hollow 
thing.  A  biological  cell,  however,  is  cytoplasm  and  nucleus. 
The  cell-wall,  which  originally  gave  the  name,  is  not  a  con- 
stant feature,  and  in  animal  cells  is  more  often  absent  than 
present. 


CHAPTER  V 

PHYLUM  ARTHROPODA,  CLASS  CRUSTACEA, 
LAW  OF  BIOGENESIS 

The  animals  belonging  to  the  phylum  Arthropoda  are 
readily  distinguished  from  all  others  by  the  presence  of 
an  exoskeleton  and  by  the  uniform  occurrence  of  paired, 
jointed  appendages  arranged  metamerically.  This  is  an 
important  phylum  numerically  for  it  contains  more  species, 
than*  all  the  rest  of  the  animal  kingdom — 400,000  or  more. 
It  includes  the  crayfishes,  crabs,  centipedes,  insects,  spiders, 
ticks,  scorpions,  etc. 

In  order  to  understand  the  general  structure  and  rela- 
tionships of  the  arthropods,  examine  Fig.  27.  This  shows 
the  structure  of  an  hypothetical  ancestor  which  may  be 
supposed  to  have  given  rise  to  all  the  existing  groups  of 
arthropods.  This  imaginary  animal  was  worm-like,  meta- 
meric,  and  had  a  tubular  digestive  system  with  a  mouth 
at  one  end  and  an  anus  at  the  other.  There  was  a  hard 
exoskeleton,  covering  every  segment  of  the  body,  and 
metameric  appendages.  The  appendages  were  rather 
broad  and  flat;  each  consisted  of  a  jointed  basal  piece  and 
two  distal  portions — hence,  was  a  biramous  appendage. 

Anterior  and  posterior  ends  were  well  differentiated. 
Several  segments  at  the  anterior  end  had  grown  together 
to  form  a  head  which  bore  a  pair  of  antennae,  a  pair  of 
eyes,  and  a  pair  of  biting  jaws  or  mandibles.  A  pair  or 
two  of  appendages,  the  maxillae,  which  were  added  to  the 
head  when  the  metameres  just  behind  fused  with  it,  had 
come  to  serve  with  the  mandibles  as  " mouth  parts"  for 
manipulating  the  food.  At  the  posterior  end  of  the  body 
the  appendages  had  perhaps  become  specialized  for  purposes 
of  reproduction. 

45 


46 


GENERAL  ZOOLOGY 


Inside  the  body  there  was  a  tubular  digestive  system 

extending  from  mouth  to  anus. 
This  began  as  a  gullet  or 
esophagus,  was  enlarged  a  little 
behind  the  head  to  form  a 
stomach,  and  continued 
through  the  rest  of  the  body 
as  a  straight  intestine.  Above 
the  alimentary  canal  there  was 
a  long  tubular  heart  which 
received  blood  through  paired 
openings  in  each  metamere  and 
pumped  it  forward.  The  ner- 
vous system  consisted  of  a 
" brain"  (the  supraesophageal 
ganglion),  dorsal  to  the  esopha- 
gus, and  a  chain  of  metameric 
masses  of  nerve  cells,  or  gang- 
lia, on  the  ventral  side  of  the 
alimentary  canal.  There  were 
nerves  from  the  ventral  chain 
of  ganglia  to  each  of  the  ap- 
pendages and  to  the  internal 
organs;  branches  from  the 
brain  supplied  the  optic  (or 
eye)  ganglion,  the  antennae, 
and  some  of  the  mouth  parts. 
There  was  a  pair  of  kidneys 
in  each  metamere  which  opened 
to  the  outside  through  pores 
in  the  exoskeleton.  Near  the 
posterior  end  of  the  body  were 
a  pair  of  tubes  (the  gonads,  or 
sex  glands),  which  shed  egg- 
or  sperm-cells  through  the 
anus. 

From  such  an  ancestor  the  five  existing  classes  of  arthro- 


ARTHROPODA  47 

pods  probably  arose  and  have  become  differentiated  by 
specializing  along  particular  lines.  This  does  not  mean 
that  modern  arthropods  have  necessarily  become  better 
than  their  remote  ancestors.  Specialization,  in  zoology, 
means  that  an  animal  is  far  from  primitive  ancestral  con- 
ditions. Some  specializations  have  been  along  progres- 
sive lines,  others  have  made  arthropods  simpler  in  struc- 
ture. Modifications  have  not  taken  place  hit  or  miss  in 
any  direction,  but  have  followed  rather  definite  lines.  The 
metameres  of  the  body  have  been  grouped  into  three 
regions — the  head,  thorax,  and  abdomen.  In  some  cases 
the  head  and  thorax  have  again  fused  so  that  there  are 
only  two  regions — the  cephalo thorax  and  abdomen.  The 
appendages  have  been  greatly  modified — in  many  instances 
one  branch  has  been  lost,  so  that  an  uniramous  condition 
has  come  about;  often  the  segments  are  lost,  combined,  or 
greatly  changed  in  shape;  sometimes  the  appendages  are 
wholly  absent  from  the  abdomen.  In  most  arthropods 
the  ancestral  number  of  mouth  parts  has  been  found  to  be 
insufficient  and  the  walking  legs  just  behind  have  been 
pressed  into  service.  In  one  group  five  pairs  of  thoracic 
legs  have  been  turned  forward  to  help  the  mouth  and  only 
three  are  left  for  walking.  The  nervous  system  has  shown 
a  tendency  to  fusion — in  some  arthropods  the  ventral 
ganglia  are  no  longer  distinct  but  have  fused  into  one  great 
mass  of  nervous  tissue.  The  sense  organs  (eyes,  ears,  etc.) 
have  become  very  complicated  in  some  cases.  The  kidneys 
seldom  retained  the  primitive  metameric  arrangement  but 
have  usually  been  restricted  to  a  single  segment  of  the 
body,  or  wholly  lost  and  replaced  by  a  new  type  of  excretory 
organ  which  opens  into  the  intestine.  The  arthropods  \ 
have,  then,  become  specialized  by  losing  parts  of  the  body,  ) 
by  adding  new  organs,  by  condensation  and  combination,  / 
and  in  other  ways — but  all  have  retained  the  same  general  / 
fundamental  plan  of  structure,  which  shows  that  they  are/ 
related.  ^ 

As  the  arthropod  line  developed,  it  early  split  into  two 


48 


GENERAL  ZOOLOGY 


groups.  One  remained  aquatic,  finally  developing  gills  for 
respiration  in  the  water,  and  gave  rise  to  the  class  Crustacea. 
All  the  other  arthropods  became  terrestrial  and  were  hence 


^^^^ 
Onycophora 


dncestral  arthropod 

FIG.  28. — A  scheme  to  show  how  the  arthropods  have  developed  from  their 
ancient  ancestor.  The  branches  are  not  intended  to  represent  actual  relation- 
ships, but  to  indicate  the  lines  of  specialization  which  have  been  followed. 


obliged  to  acquire  the  ability  to  breathe  air.  Most  of  the 
air-breathing  arthropods  have  developed  a  branching 
system  of  minute  tubes,  the  tracheae,  which  carry  air  to 


ARTHROPODA  49 

all   parts   of   the   body.     We    can,    therefore,    make    two 
divisions  of  the  Phylum  Arthropoda: 

Division    I.     Branchiata — breathing    in    water,    through    the    general 

surface  of  the  body  or  through  gills. 
Division  II.     Tracheata — breathing  air,  through  tracheae  or  in  some 

other  way. 

The  first  division  contains  only  the  class  Crustacea,  and 
the  Tracheata  therefore  include  the  other  four  classes. 
Two  of  the  latter  have  retained  the  ancestral  worm-like 
form,  with  metameric  appendages,  but  are  easily  dif- 
ferentiated from  each  other  because  one  class  (Myriapoda) 
has  lost  the  primitive  metameric  kidneys  completely  and 
the  other  has  retained  them  (Onycophora).  The  other 
two  classes  have  departed  from  the  primitive  worm-like 
form  and  are  highly  specialized.  They  have  lost  the 
appendages  on  the  posterior  end  of  the  body;  lost  the 
metameric  kidneys  and  developed  a  new  type  of  excretory 
organ  (the  Malpighian  tubules);  and  become  specialized 
in  many  other  ways.  One  of  these  last  two  classes  (Insecta) 
has  kept  only  six  walking  legs  and  has  developed  wings, 
the  other  (Arachnida)  has  retained  eight  walking  legs  and 
never  possesses  wings.  We  may  at  present,  therefore, 
classify  the  Phylum  Arthropoda  on  the  following  basis: 

Division  I.     Branchiata.     Aquatic,  breathe  through  skin  or  gills. 

Class  1.     Crustacea.     Aquatic  arthropods,   with  primitive  but 

nori-metameric  kidneys. 

Division  II.     Tracheata.     Terrestrial  (or  aquatic),  breathe  air,  usually 
through  tracheae. 

Class  2.  Onycophora.  Worm-ljke,  with  paired  metameric  kid- 
neys throughout  the  body. 

Class  3.  Myriapoda.  Worm-like;  excretion  through  Malpig- 
hian tubules  which  .open  into  the  intestine  just  behind  the 
stomach. 

Class  4.  Insecta.  Three  pairs  of  walking  legs;  one  or  two  pairs 
of  wings  usually  present;  no  metameric  kidneys,  Malpighian 
tubules  present. 

Class  5.  Arachnida.  Four  pairs  of  walking  legs;  no  wings; 
Malpighian  tubules  present. 


50 


GENERAL  ZOOLOGY 


The  relationships  of  the  classes  of  arthropods  are  shown 
in  Fig.  28.  Let  us  now  consider  each  of  them  in  more  detail. 
The  remainder  of  this  chapter  will  be  devoted  to  the 
Crustacea. 

CLASS  1.     CRUSTACEA 

This  class  may  be  defined  as  including  arthropods  which 
live  for  the  most  part  in  water,  and  usually  breathe  through 
gills.  We  can  perhaps  best  get  an  idea  of  one  of  these 
animals  by  considering  the  activities  of  an  example  which  is 
familiar  to  everyone — the  common  crayfish,  or  crawfish, 


FIG.  29.— Crayfishes. 

found  in  fresh  water  the  world  over.  The  daily  life  of  this, 
or  any  other,  animal  is  shaped  primarily  for  three  ends— 
self-maintenance,  self-protection,  and  race  preservation. 
To  use  the  words  of  Alcock — "the  three  great  exigencies: 
to  find  something  to  eat,  to  avoid  being  oneself  eaten,  and 
to  disseminate  one's  species,  give  rise  to  a  perpetual  struggle 
in  which  the  fittest  is  successful. " 

THE  CRAYFISH,  Cambarus  virilis  Hagen 

Self -maintenance. — Crayfishes  (Fig.  29)  eat  nearly  any- 
thing that  is  available.     They  prefer  animal  food,  and  lurk 


CRUSTACEA  51 

in  sheltered  nooks  waiting  for  some  unwary  fish  or  insect 
to  swim  by.  They  catch  such  animals  in  their  big  pin- 
chers, or  chelipeds,  and  tear  them  to  pieces.  If  living 
animal  food  fails,  crayfishes  eat  plants,  or  any  sort  of  dead 
organic  matter — and  are  hence  good  scavengers.  Fine 
particles  of  food  are  retained  by  the  little  bristles  along  the 
margins  of  the  chelipeds  and  the  mouth  parts,  so  that  noth- 
ing of  value  is  lost. 

A  crayfish  is  well  equipped  with  sense  organs  to  find  its 
food.  There  are  two  pairs  of  antennae  at  the  anterior  end 
of  the  body  and  the  shorter  pair,  the  antennules,  is  not  only 
provided  with  sensory  bristles  for  receiving  touch  stimuli, 
but  possesses  organs  of  smell  as  well.  A  crayfish  is  able 
to  taste  with  the  nerve  endings  on  the  mouth  parts.  The 
compound  eyes  are  set  on  stalks  so  that  they  can  be  moved 
about  in  different  directions.  Each  is  made  up  of  hundreds 
of  minute  but  complete  eyes.  Together  these  little  eyes 
permit  a  crayfish  to  see  a  complete  but  somewhat  broken 
picture  of  its  surroundings — a  mosaic  image.  If  a  crayfish 
is  blind  in  one  of  its  simple  eyes,  there  is  a  space  in  its  field 
of  vision  where  nothing  is  visible. 

Once  the  food  is  secured,  it  must  be  converted  into  living 
substance  or  modified  so  as  to  serve  as  fuel  to  run  the  living 
machine.  It  is  pulled  apart  by  the  chelipeds,  torn  into 
shreds  by  the  mouth  parts,  then  passes  down  into  the  stom- 
ach, where  it  is  further  chewed  by  three  strong  projecting 
teeth  and  strained  between  little  bristles.  It  is  mixed  with 
the  digestive  secretion  from  the  "  liver"  and  finally  absorbed 
through  the  walls  of  the  intestine.  What  cannot  be  digested 
and  absorbed  passes  out  of  the  anus  as  faeces.  The  digested 
food  which  passes  through  the  wall  of  the  alimentary  canal 
enters  great  blood  spaces,  the  sinuses,  and  ultimately  reaches 
the  small  muscular  heart,  in  the  middle  of  the  back,  which 
pumps  blood  to  all  parts  of  the  body.  The  heart  works  only 
when  it  contracts.  It  is  passive  during  expansion,  being 
stretched  out  by  little  elastic  strands  which  lead  from  it  to 
the  walls  of  the  chamber  in  which  it  lies.  The  blood  of  the 


52  GENERAL  ZOOLOGY 

crayfish  is  colorless,  but  carries  food,  oxygen,  and  waste 
products. 

A  crayfish  labors  under  one  great  disadvantage — the 
exoskeleton  is  so  rigid  that  it  cannot  grow  from  day  to  day, 
as  do  animals  with  pliable  skins.  This  shell  is  an  admir- 
able protection  but  prohibits  growth.  This  difficulty  is 
surmounted  by  a  periodical  casting  of  the  entire  exo- 
skeleton— including  the  lining  of  the  esophagus  and  stom- 
ach. After  such  a  moult  the  crayfish  is  a  "soft  shell"  for  a 
day.  During  this  period  the  body  absorbs  a  great  deal  of 
water,  and  there  is  very  rapid  increase  in  size.  The  shell 
then  hardens  and  no  more  growth  can  take  place  until  the 
next  moult.  Just  before  the  shell  is  shed  a  little  stony 
secretion,  the  gastrolith,  is  formed  on  either  side  of  the 
stomach.  This  is  a  reserve  supply  of  lime  which  is  rapidly 
transferred  by  the  blood  to  the  new  shell  after  moulting. 

The  crayfish  has  a  special  respiratory  system  for  securing 
oxygen,  which  consists  of  a  number  of  plume-like  gills  along 
the  sides  of  the  thorax.  These  delicate  breathing  organs 
are  protected  by  the  hard  shell,  or  carapace,  which  covers 
the  cephalothorax.  One  of  the  mouth  parts  has  a  little 
paddle  which  projects  into  the  respiratory  chamber  and  by 
its  movements  keeps  a  current  of  water  passing  forward  over 
the  gills  and  out  under  the  head.  The  gills  are  full  of 
moving  blood  which  receives  oxygen  from  the  passing 
water.  The  carapace  fits  over  the  thorax  much  as  a  man's 
coat  covers  his  body,  and  the  gill  chamber  is  kept  from 
getting  dirty  by  little  bristles  along  its  edge  which  strain 
the  incoming  water. 

The  excretions  resulting  from  general  metabolism  in  the 
crayfishes'  body  are  eliminated  through  the  " green  gland" 
— a  form  of  kidney  consisting  of  a  long  tube  which  is  ex- 
panded into  a  bladder  near  the  discharg;ng  end.  Excre- 
tions are  absorbed  from  the  blood  by  the  green  gland  and 
passed  out  through  a  pore  which  opens  through  the  basal 
segment  of  the  second  antenna.  There  is  only  one  pair  of 
green  glands  and  these  are  wholly  within  the  head. 


CRUSTACEA  53 

From  what  has  been  said,  it  is  apparent  that  a  crayfish 
is  well  able  to  find  substances  fit  for  food  and  to  reject  that 
which  is  unfit.  Elaborate  and  specialized  systems  of 
organs  for  digestion,  assimilation,  circulation,  respiration, 
and  excretion  are  present. 

Self -protection. — The  solitary  crayfish  has  many  ene- 
mies— fish  which  seek  to  devour  it,  parasites  which  suck 
its  blood  or  sap  its  vitality  by  infesting  the  interior  of  the 
body,  minute  moulds  or  bacteria  which  cause  diseased 
conditions.  How  does  it  escape  these? 

Larger  enemies  are  usually  avoided  by  hiding  in  crevices 
between  stones  or  water  plants.  If  a  crayfish  is  dislodged 
from  its  retreat,  it  swims  quickly  backward  by  flipping  the 
powerful  tail  fin  and  often  finds  a  new  hiding  place  (Fig.  29). 
But  if  captured,  it  does  not  surrender  without  a  struggle. 
The  great  claws  inflict  painful  wounds  and  the  smaller  legs 
scratch — the  cornered  animal  fights  literally  "  tooth  and  toe- 
nail,"  until  it  is  bested  or  escapes.  Even  if  a  leg  or  two  is 
lost  the  damage  is  not  serious  for  a  new  one  is  formed 
beneath  the  exoskeleton,  and  after  the  next  moult  is  nearly 
as  large  as  before.  On  account  of  the  large  spaces  within 
the  body,  a  crayfish  easily  bleeds  to  death  if  the  body  wall 
is  punctured.  In  the  appendages,  however,  excessive 
bleeding  is  prevented  by  a  special  arrangement.  A  crushed 
leg  is  thrown  off  at  a  " casting  joint"  near  its  base,  where 
there  is  a  very  small  aperture  from  the  blood  sinuses  and 
the  opening  quickly  closes. 

The  color  of  a  crayfish  is  usually  more  or  less  like  the 
surroundings,  and  can  be  changed  somewhat  by  the  con- 
traction or  expansion  of  little  pigment  cells  in  the  skin. 
Such  " protective  resemblance"  makes  it  less  conspicuous. 

The  insidious  enemies  which  attack  the  crayfish  as  para- 
sites are  wardedAoff  chiefly  by  sanitary  measures.  Scrupu- 
lous cleanliness  is  observed.  There  are  special  combs  and 
bristles  on  the  appendages  for  cleaning  the  exoskeleton 
and  it  is  continually 'scraped  and  polished.  If  there  is  a 
break  in  the  outside  covering,  the  way  is  opened  for  various 


54  GENERAL  ZOOLOGY 

moulds  and  bacteria  to  gain  an  entrance.  These  are  fought 
off  by  little  motile  cells,  the  amoebocytes,  until  the  wound 
can  heal. 

But  a  crayfish  must  struggle  against  other  things  besides 
aggressive  enemies  and  stealthy  parasites.  It.  is  obliged 
to  meet  the  changing  conditions  of  environment  so  as  not 
to  be  destroyed  by  storms,  or  cold,  or  other  unfavorable 
variations.  Crayfishes  are  resistant  to  low  temperatures 
and  live  a  slow,  yet  active,  life  beneath  the  ice  where  there 
is  a  winter  season.  They  do  not  fare  so  well  if  the  water 
becomes  too  warm,  and  are  therefore  less  abundant  in  the 
tropics  than  elsewhere.  They  can  live  for  a  considerable 
time  out  of  water,  particularly  if  they  can  rest  in  some 
damp  place,  as  among  fallen  leaves. 

Crayfishes  quickly  succumb  to  foul  water  and  sometimes 
migrate  over  the  land  to  escape  from  a  stagnant  pool.  If 
the  pond  in  which  they  have  been  living  dries  up,  they 
burrow  into  the  soil  and  remain  quiet  until  water  is  again 
available.  Some  crayfishes  have  become  expert  diggers 
and  make  burrows  far  from  water;  in  fact,  some  enter  the 
water  only  during  the  breeding  season.  Such  species  often 
do  damage  by  making  holes  in  fields  or  by  perforating  dykes. 
They  are  easily  killed  by  dropping  unslaked  lime  down  their 
burrows  and  closing  them  tightly. 

Race  Preservation. — Crayfishes  are  of  separate  sexes. 
Mature  males  are  readily  distinguished  from  females  by 
their  larger  chelipeds  and  narrower  abdomens.  The  repro-*J 
ductive  gland,  or  testis,  of  the  male  lies  in  the  posterior 
portion  of  the  thorax  and  is  connected  by  two  tortuous 
ducts,  the  vasa  deferentia,  with  small  pores  in  the  basal 
joints  of  the  last  pair  of  walking  legs.  The  first  two  pairs 
of  abdominal  appendages  in  the  male  are  peculiarly  modi- 
fied so  that  they  may  be  used  in  transferring  sperm  to  the 
female. 

During  the  breeding  season  the  males  move  about 
actively  in  search  of  females.  When  one  is  encountered, 
a  little  packet  (the  spermatophore)  full  of  sperm  cells  is 


CRUSTACEA  55 

transferred  to  her.  In  some  species  the  females  have  a 
small  pouch  in  the  exoskeleton  (the  annulus)  into  which 
the  spermatophore  is  thrust  by  the  male. 

A  female  has  an  ovary  in  the  thorax  which  connects  by 
two  short  ducts  with  openings  in  the  basal  segments  of  the 
third  pair  of  walking  legs.  After  receiving  a  spermato- 
phore she  prepares  with  great  care  to  lay  eggs.  The 
abdomen  and  its  appendages  are  cleaned  for  two  or  three 
days  by  scraping  them  with  the  last  pair  of  walking  legs. 
The  female  then  lies  on  her  side,  bends  the  abdomen  a  little, 
and  secretes  a  gelatinous  apron  over  the  underside  of  the 
body  (Fig.  29).  The  eggs  are  then  ejected  from  the  opening 
in  the  third  legs;  the  spermatophore  dissolves  and  the 
sperm  cells  "fertilize"  the  eggs  inside  the  gelatinous  apron. 

The  jelly  of  the  apron  soon  shrivels  and  breaks  up;  the 
eggs  becoming  fixed  to  the  abdominal  appendages  by  little 
stalks.  The  mother  crayfish  takes  good  care  of  the  eggs; 
they  are  kept  clean  and  frequently  aerated  by  elevating 
the  abdomen  and  waving  the  appendages  which  bear  them 
(Fig.  29).  There  is  yolk  material  stored  within  each  egg- 
shell to  nourish  the  developing  embryo  until  it  hatches. 
When  the  little  crayfish  emerges,  it  is  attached  to  the  old 
egg-shell  by  a  slender  filament  which  issues  from  the  anus. 
It  remains  thus  fastened  to  the  mother  even  after  its  first 
moult  (Fig.  30),  but  when  the  skin  is  shed  the  second  time, 
the  little  creature  is  free  and  soon  able  to  shift  for  itself. 
A  female  crayfish  carries  from  three  to  six  hundred  eggs 
at  a  time  and  after  becoming  mature  usually  breeds 
annually.  A  crayfish  usually  lives  to  be  six  or  seven  years 
old. 

A  mature  crayfish  reacts  to  its  surroundings  so  as  to  be 
successful  as  long  as  possible,  and  thus  continue  its  race. 
How  much  mental  ability  does  it  display  in  responding 
to  the  stimuli  which  it  receives  ?  Very  little.  Its  behavior 
is  largely  made  up  of  reflexes  and  instinctive  activities. 
If  a  large  object  appears  in  its  field  of  vision,  the  crayfish 
crouches  down  into  its  crevice;  if  a  small  animal  moves, 


56  GENERAL  ZOOLOGY 

it  rushes  forth,  seizes  the  intruder  and  tries  to  eat  it.  When 
hunger  is  satisfied  the  crayfish  sits  in  its  lair  without  any 
movement,  except  for  respiration,  for  days  at  a  time.  Its 
activities  are  for  the  most  part  strictly  utilitarian.  Yet  a 
crayfish  has  some  ability  to  profit  by  experience.  If  a  large 
shadow  is  cast  over  one  repeatedly,  it  first  avoids  the  shadow 
at  each  repetition  and  puts  itself  in  an  attitude  for  defense, 
but  finally  learns  to  ignore  this  new  thing  which  has  come 
into  its  life.  Professor  Yerkes  taught  a  crayfish  to  go 


Fio.  30. — A  young  crayfish  attached  to  a  portion  of  one  of  its  mother's 
abdominal  appendages.  Until  after  its  second  moult  the  young  crayfish  is 
firmly  fastened  by  an  "anal  filament."  (Adapted  from  Andrews.) 

through  a  simple  labyrinth  where  it  had  equal  chance  to 
enter  a  blind  alley  or  to  go  through  an  opening  into  the 
water.  At  first  it  took  the  wrong  course  half  of  the  time, 
but  after  two  hundred  and  fifty  trials  made  no  more 
mistakes. 

If  we  compare  a  crayfish  with  the  ancestral  arthropod 
described  at  the  beginning  of  this  chapter,  we  see  that  it 
is  specialized  in  several  respects.  The  abdomen  is  the 
most  primitive  part  of  the  body,  being  clearly  metameric 
with  a  pair  of  biramous  appendages  on  each  segment.  The 
thorax  is  overgrown  by  the  great  carapace  and  shows  evi- 


CRUSTACEA  57 

dence  of  segmentation  only  on  the  ventral  side.  The 
thoracic  appendages  are  specialized;  most  of  them  have  lost 
the  outer  branch  and  become  uniramous.  Yet  with  careful 
examination  in  the  laboratory  we  can  demonstrate  that  a 
fundamentally  similar  plan  of  structure  is  shown  by  all 
of  the  appendages  on  the  body  of  the  crayfish,  except  the 
eye.  If  time  permitted  it  would  be  possible  to  show  many 
other  ways  in  which  the  crayfish  is  specialized,  but  we  must 
pass  on  to  consider  some  of  its  relatives. 

CLASSIFICATION  OF  CRUSTACEA 

The  Crustacea  have  in  their  specialization  followed  various 
lines  and  we  can  divide  the  class  into  two  great  groups,  or 
subclasses,  and  a  large  number  of  orders.  It  will  probably 
not  be  profitable  for  the  student  to  try  to  learn  the  follow- 
ing list,  but  it  is  given  in  full  in  order  that  one  may  be  able 
to  see  for  himself  what  a  degree  of  diversity  has  been  at- 
tained in  ihigL  most  primitive  class  of  arthropods,  contain- 
ing only  about  16,000  living  species.  Before  we  pass  to 
the  table  of  classification  it  will  be  well  to  point  out  the 
chief  lines  which  specialization  has  followed : 

1.  The  heart  has  been  greatly  shortened  so  that  in  some  crustaceans 
it  occupies  only  one  or  two  metameres. 

2.  The  kidneys  are  always  restricted  to  one  pair. 

3.  Gills  may  be  developed  along  the  sides  of  the  thorax,  or  on  the 
thoracic  or  abdominal  appendages. 

4.  The  primitive  biramous  appendage  is  frequently  modified  by  the 
dropping  out  of  the  external  ramus  or  by  the  loss  of  segments. 

5.  A  carapace  may  be  developed.    This  may  cover  only  a  part  or 
the  whole  of  the  thorax,  or  be  large  enough  to  enclose  the  entire  body. 

6.  There  is  a  tendency  to  acquire  mouth  parts  at  the  expense  of  the 
walking  legs. 

7.  The  appendages  are  often  lost  on  the  abdomen. 

8.  The  eyes  sometimes  become  stalked. 

With  these  points  in  mind  let  us  pass  on  to  the  classifica- 
tion of  the  Crustacea. 

Trilobita. — This   group   of  extinct   arthropods  was  ex- 


58 


GENERAL  ZOOLOGY 


tremely  variable,  the  number  of  body  segments  ranging 
from  four  to  thirty-one.  Antennae,  compound  eyes,  and 
biramous  appendages  were  present.  All  trilobites  were 
marine  and  none  have  been  alive  for  the  past  several 
million  years.  Recent  Crustacea  are  classified  as  follows : 


DiHerogammarus  Combarus  Ca/linectes 

FIG.  31. — Examples  of  the  Class  Crustacea. 

Subclass  I.    Entomostraca. — Number  of  metameres  variable;  kidney 
opens  in  base  of  maxillae;  no  gastric  mill  in  stomach. 

Order   1.  PHYLLOPODA. — With  ten  to  thirty  pairs  of  leaf -like 

feet;  a  carapace  often  present.     Examples:  Eubranchipus, 

Estheria  (Fig.  31). 
Order  2.  CLADOCERA. — Second  antenna?  large,   biramous,  and 

used  for  swimming;  body  usually  enclosed  in  a  bivalve  shell. 

Examples:  Daphnia,  Bosmina  (Fig.  31). 
Order  3.  OSTRACODA. — Only  seven  pairs  of  appendages.     Body 

completely  enclosed  in  a  bivalve  shell.     Example:  Cypris 

(Fig.  32). 


CRUSTACEA  59 

Order  4.  COPEPODA. — With  large  uniramous  antennae  which  are 
used  for  swimming;  abdomen  without  metameric  appendages. 
Examples:  Cyclops  (Fig.  31),  Diaptomus. 

Order  5.  CIRRIPEDIA. — Fixed  or  parasitic;  body  enclosed  in  a 
calcareous,  non-metameric  shell;  barnacles.  Example: 
Balanus  (Fig.  31). 

Subclass  2.  Malacostraca. — Usually  of  large  size;  with  five  metameres 
in  the  head,  eight  in  the  thorax,  and  six  in  the  abdomen;  with  a 
grinding  apparatus,  the  gastric-mill,  in  the  stomach. 

Order  6.  NEBALIACEA. — Small  shrimp-like,  marine  crustaceans, 
with  an  extra  metamere  in  the  abdomen;  carapace  well 
developed. 

Order  7.  ANASPIDACEA. — With  distinct  thoracic  segments, 
stalked  eyes,  and  no  carapace. 

Order  8.  MYSIDACEA. — Small  shrimp-like  crustaceans;  with 
biramous  thoracic  legs,  and  a  large  carapace. 

Order  9.  CUMACEA. — Malacostraca  without  abdominal  append- 
ages, with  a  small  carapace  and  four  or  five  free  thoracic 
segments. 

Order  10.  TANAIDACEA. — Malacostraca  with  all  but  two  thoracic 
segments  free;  respiratory  chambers  on  sides  of  thorax. 

Order  11.  ISOPODA. — Body  usually  broad  and  flat;  no  carapace; 
seven  free  thoracic  segments.  Examples:  Asellus,  Porcellio 
(Fig.  31). 

Order  12.  AMPHIPODA. — Body  usually  laterally  compressed; 
with  seven  free  thoracic  segments;  usually  with  three  pairs 
of  appendages  fitted  for  jumping  at  the  posterior  end. 
Example:  Dikerogammarus  (Fig.  31). 

Order  13.  EUPHAUSIACEA. — Thorax  covered  by  carapace;  with 
respiratory  chambers  at  sides;  no  thoracic  legs  serve  as 
mouth  parts. 

Order  14.  DEC  APOD  A. — With  first  three  pairs  of  thoracic  ap- 
pendages specialized  as  mouth  parts;  ten  walking  legs; 
carapace  covers  thorax;  stalked  eyes. 

Suborder  1.  Natantia. — Slender,  laterally  compressed;  ab- 
domen well  developed  and  provided  with  appendages.  Ex- 
amples: Cambarus  (Fig.  31),  lobster,  shrimps,  prawns. 
Suborder  2.  Reptantia. — Flattened,  short;  abdomen  small  and 
folded  forward  under  thorax.  Examples:  Callinectes  (Fig. 
31),  crabs. 

Order  15.  STOMATOPODA. — Five  pairs  of  thoracic  limbs  serve  as 
mouth  parts;  six  biramous  walking  legs;  carapace  covers 
only  part  of  thorax. 


60  GENERAL  ZOOLOGY 

LAW  OF  BIOGENESIS 

The  Crustacea  may  well  serve  to  illustrate  one  of  the 
great  principles  of  zoology — are  particularly  appropriate, 
in  fact,  because  the  law  in  question  was  first  discovered 
while  the  classification  of  this  class  was  being  investigated. 
The  "Law  of  Biogenesis, "  otherwise  known  as  the  " Re- 
capitulation Theory,"  holds  that  ontogeny  repeats  phy- 
logeny — i.e.,  that  each  individual  animal  in  its  develop- 
ment repeats  the  stages  through  which  its  race  has  passed 
in  its  evolution.  Huxley  once  put  the  idea  briefly  by 
saying:  " Every  animal  climb.s  its  own  ancestral  tree." 

This  law  has  been  of  great  value  to  zoologists  in  helping 
to  work  out  the  true  relationships  of  animals.  For  example, 
because  man,  in  his  early  embryonic  development,  has  gill- 
like  structures  at  the  sides  of  his  neck  and  a  fish-like  form, 
it  is  believed  that  he  came  from  what  was  once  a  fish-like 
ancestor  which  lived  in  the  water.  Furthermore,  it  seems 
reasonable  to  believe  that  all  vertebrates  came  from  a 
common  ancestry,  because,  up  to  a  certain  point,  the  de- 
velopmental history  of  all  is  very  similar.  The  embryonic 
record  is  often  dim,  or  warped.  There  are  instances  where 
stages  which  should  come  later  are  sometimes  pushed  ahead, 
and  embryonic  animals  sometimes  acquire  new  specializa- 
tions which  their  remote  ancestors  did  not  possess,  but  the 
great  landmarks  in  the  embryonic  records  generally  keep 
their  resemblance  to  the  racial,  or  phylogenetic,  history  so 
that  the  story  is  readable. 

The  value  of  the  Law  of  Biogenesis  is  well  illustrated  in 
the  Crustacea.  The  barnacles,  for  example,  were  until  a 
short  time  ago  of  very  questionable  relationships.  Some 
zoologists  believed  that  they  should  be  placed  with  the 
molluscs  because  they  had  a  hard  shell,  others  that  they 
should  be  classed  with  the  worms,  but  no  one  thought  of 
relating  them  to  crustaceans.  Then  it  was  discovered  that 
many  entomostracans  hatch  from  the  egg  as  a  little  six- 
legged  larva,  the  nauplius.  This  nauplius  larva  swims 


LAW  OF  BIOGENESIS 


61 


FIG.  32. — Stages  in  the  development  of  three 
crustaceans  illustrating  the  Law  of  Biogenesis,  or 
the  Law  of  Recapitulation.  On  the  first  line  three 
eggs  are  shown.  On  the  second  the  nauplei  are  rep- 
resented which  hatched  from  the  respective  eggs 
above.  The  middle  nauplius  is  a  good  example  of 
"acceleration  in  development,"  for  it  is  already  en- 
closed in  a  little  shell  when  it  hatches.  The  larvae  of 
its  remote  ancestors  probably  had  no  such  shell  and 
this  feature,  therefore,  distorts  the  embryological 
record  by  coming  earlier  than  in  the  past.  The  third 
line  represents  the  adult  stages  of  the  first  two  crus- 
taceans (A3,  J33)  and  what  was  at  one  time  probably  the  adult  stage  in  the  an- 
cestors of  the  third  (C3).  The  lower  figure  shows  that  the  free  stage  shown 
in  the  line  above  has  been  succeeded  by  a  fixed,  or  sessile,  condition.  This 
last  change  is  accompanied  by  some  degeneration.  A,  Cyclops;  B,  Cypris;  Ct 
Balanus. 


62  GENERAL  ZOOLOGY 

around  in  the  water  until  its  first  moult,  when  it  acquires 
more  legs  and  the  form  characteristic  for  its  particular 
species.  After  the  nauplius  stage  some  kinds  of  crusta- 
ceans become  Copepoda,  others  Ostracoda,  etc.,  but  all 
agree  pretty  closely  up  to  the  time  when  their  nauplius 
skin  is  shed.  When  the  embryology  of  a  barnacle  came  to 
be  studied — it  hatched  from  the  egg  as  a  nauplius!  Since 
this  fact  was  discovered  zoologists  have  not  questioned  the 
right  of  barnacles  to  be  known  as  relatives  of  Cyclops, 
Daphnia,  and  other  crustaceans.  The  life  history  of  a 
barnacle  indicates  that  its  race  developed  to  a  free  swim- 
ming stage  comparable  to  the  adult  forms  of  other  entomos- 
tracans;  then  took  up  a  sessile  existence,  specialized  along 
new  lines,  and  even  degenerated  somewhat  because  it  led 
an  inactive  life.  Fig.  32  shows  larval  and  adult  stages  of 
the  barnacle  and  two  of  its  relatives. 


CHAPTER  VI 

PHYLUM  ARTHROPODA,  CLASSES— ONYCO- 
PHORA,  MYRIAPODA,  INSECTA 

CLASS  2.      ONYCOPHORA 

The  sole  representatives  of  this  class  of  arthropods  are 
the  sixty-odd  species  of  the  genus  Peripatus.  Peripati 
occur  only  in  certain  parts  of  the  world — Northern  Africa, 
Northern  South  America,  Mexico,  and  some  of  the  South 
Pacific  islands.  These  peculiar  worm-like  animals  live  in 
rotten  logs,  in  crevices  in  rocks,  among  fallen  leaves,  and 


FIG.  33. — Peripatus  entangling  a  cockroach  in  sticky  threads  squirted  from  two 
papillae  beneath  its  head. 

in  similar  situations.  Their  food  consists  of  small  insects, 
spiders,  etc.  They  entangle  their  prey  in  little  sticky 
threads  which  are  ejected  from  glands  on  two  papillae 
beneath  the  head  (Fig.  33). 

The  body  of  Peripatus  is  cylindrical  and  worm-like.  It 
has  from  seventeen  to  forty  pairs  of  small  legs,  a  pair  of 
jointed  antennae,  and  an  eye  just  beneath  each  antenna. 
A  single  pair  of  appendages  have  been  modified  to  serve 
as  mouth  parts — these  act  as  jaws  and  tear  the  food  to 
pieces.  Most  species  of  Peripatus  bring  forth  the  young 
alive.  There  is  no  metamorphosis — the  young  resembling 
the  parents  in  their  general  structure. 

63 


64 


GENERAL  ZOOLOGY 


Peripatus  is  of  great  zoological  interest  because  it  is  a 
1 '  missing  link."  It  has  many  points  of  marked  resemblance 
to  annelid  worms,  and  at  the  same  time  possesses  character- 
istics which  make  it  unquestionably  belong  to  the  Phylum 
Arthropoda.  The  annelidan  affinities  are  shown  by:  (1) 
the  paired  metameric  nephridia  (kidneys) ;  (2)  the  presence 
of  cilia  in  the  reproductive  organs;  and  (3)  the  general 
arrangement  of  the  chief  systems  of  organs.  Arthropod 
characteristics  are:  (1)  appendages  modified  as  jaws;  (2) 
a  body  cavity  mostly  converted  into  large  blood  sinuses; 
(3)  the  presence  of  tracheae  for  breathing;  and  (4)  the  paired 
jointed  appendages.  Peripatus  closely  resembles  the  hypo- 
thetical ancestral  arthropod  described  at  the  beginning  of 
Chapter  5  (Fig.  27). 


CLASS  3.     MYRIAPODA 

This  class  includes  the  centipedes  and  millipeds  (Fig. 
34).     These  animals  are  readily  distinguished  from  other 


FIG.  34. — At  the  left  a  centipede  eating  a  fly;  at  the  right  millipedes  resting  and 

eating  a  leaf. 

arthropods  by  the  following  characteristics:  (1)  a  distinct 
head  with  one  pair  of  antennae  and  simple  eyes;  (2)  a  long 
body  composed  of  many  free  similar  segments ;  (3)  tracheae 
for  breathing;  and  (4)  Malpighian  tubules  for  excretory 
organs.  The  myriapods  are  divided  into  three  orders: 

Order  1.     Diplopoda. — These  are  the  millipeds.     As  the 
name  of  the  order  indicates,  they  have  two  pairs  of  jointed 


.  MYRIAPODA  65 

legs  on  each  segment;  also,  on  the  head,  a  pair  of  antennae, 
a  pair  of  mandibles,  and  one  pair  of  maxillae.  Millipeds 
are  usually  cylindrical  in  form,  but  some  species  are  flat. 
They  are  slow-moving  sluggish  creatures  whose  food  con- 
sists entirely  of  vegetation.  They  often  do  damage  to 
crops  or  greenhouse  plants.  Julus  and  Polydesmus  are 
common  genera. 

Order.  2.  Chilopoda. — This  order  includes  the  centi- 
pedes, which  have  only  one  pair  of  legs  on  each  metamere. 
The  body  is  flat  and  has  maxillipeds,  which  are  a  pair  of 
modified  legs,  in  addition  to  the  antennae,  mandibles,  and 
maxillae  mentioned  as  occurring  in  the  last  order.  The 

Heart  Malpighian  tubule 

Optic  gangl,on      £nteron 
Antenna         /Brain 


35. — A  centipede  with  the  body  wall  and  appendages  of  the  left  side  re- 
moved to  show  the  internal  organs.     Compare  with  Fig.  27. 

maxillipeds  in  centipeds  are  poison  claws.  They  have 
little  openings  at  their  tips  which  connect  with  poison 
glands  within  the  body.  The  poison  claws  are  used  in 
killing  small  insects  and  spiders  for  food.  The  large 
tropical  centipedes  inflict  painful  bites,  and  have  even  been 
known  to  kill  children. 

Order  3.  Symphyla. — These  are  small  myriapods  with 
only  twelve  pairs  of  legs.  They  are  rare,  but  are  of  some 
theoretical  interest  because  they  resemble  the  most  primi- 
tive insects  (Aptera)  in  the  structure  of  their  mouth  parts 
and  in  some  other  features. 

Many  of  the  myriapods  remain  with  their  eggs  until  they 
hatch.  Some  species  build  little  nests  by  hollowing  out  a 
space  in  the  ground  or  in  a  rotten  log,  or  by  plastering  little 
pellets  of  mud  together  to  make  a  dome-shaped  case. 


66  GENERAL  ZOOLOGY 

The  Myriapoda  (Fig.  35)  show  many  points  of  progress 
when  compared  with  our  hypothetical  ancestral  arthropod 
(Fig.  27).  The  metameric  kidneys  have  been  lost  and 
replaced  by  Malpighian  tubules;  tracheae  have  been  de- 
veloped for  breathing;  the  appendages  are  all  uniramous 
except  one  pair  of  mouth  parts;  and  the  different  orders 
show  a  progressive  tendency  to  make  walking  legs  into 
mouth  parts. 

CLASS  4.     INSECTA 

The  class  Insecta,  once  known  as  the  Hexapoda  (six 
legs),  contains  an  enormous  number  of  species — exceeding 
all  the  rest  of  the  animal  kingdom.  The  class  is  not  only 
large  but  versatile  as  well.  Insects  show  all  sorts  of  peculiar 
adaptations  for  specialized  modes  of  life.  They  are  found 
in  all  parts  of  the  world  and  live  in  almost  every  conceiv- 
able habitat.  They  vary  in  size  from  a  twentieth  of  an 
inch  to  nearly  seven  inches  in  length. 

The  body  of  an  insect  has  three  distinct  regions:  head, 
thorax,  and  abdomen.  The  head  bears  a  single  pair  of 
antennae,  usually  two  compound  eyes,  three  simple  eyes 
or  ocelli,  and  four  different  kinds  of  mouth  parts — the 
labrum,  which  is  single  and  not  one  of  the  series  of  matemeric 
appendages;  the  mandibles,  maxillae,  and  labium,  which 
are  paired.  The  thorax  is  always  composed  of  three  seg- 
ments— prothorax,  mesothorax,  metathorax.  Each  seg- 
ment is  protected  by  four  exoskeletal  plates — a  dorsal 
tergum,  a  ventral  sternum,  and  two  lateral  pleura.  Each 
thoracic  metamere  bears  a  pair  of  walking  legs,  and  each 
of  the  last  two  frequently  bears  a  pair  of  wings.  The  ab- 
domen typically  consists  of  eleven  free  segments  which  are 
without  appendages,  except  accessory  reproductive  organs 
or  a  sting  at  the  posterior  end. 

The  mouth  parts  vary  greatly  but  conform  mostly  to 
two  types:  (1)  for  biting,  as  in  a  beetle;  or  (2)  for  sucking, 
as  in  a  bug.  Some  insects,  however,  like  the  honey  bee, 
may  have  very  specialized  mouth  parts  which  may  be  used 


INSECTA  67 

for  both  biting  and  sucking.  The  walking  legs  have  five 
parts:  a  proximal  coxa,  often  fixed  immovably  to  the 
sternum  to  which  it  is  attached;  a- short  trochanter;  a  long 
femur;  a  slender  tibia;  and  a  jointed  tarsus  which  is  usually 
provided  with  little  hooks  or  pads  at  its  free  end.  The  legs 
may  be  adapted  in  various  ways  for  grasping,  swimming, 
digging,  leaping,  or  other  purposes.  The  wings  arise  as 
outgrowths  from  the  two  posterior  thoracic  segments. 
They  are  chiefly  of  two  types:  broad  for  sailing,  as  on  a 
butterfly;  and  narrow  for  rapid  propulsion,  as  on  a  house 
fly.  They  often  bear  scales  or  hairs.  They  may  be  thin 
and  membranous,  thick  and  heavy  for  protection,  or  vary 
in  other  ways.  The  little  " veins"  which  traverse  insects' 
wings  are  not  primarily  for  carrying  blood,  but  are  thicken- 
ings serving  as  supporting  or  skeletal  structures. 

The  inside  of  an  insect's  body  is  filled  with  digestive, 
reproductive,  respiratory,  circulatory,  and  excretory  organs. 
The  alimentary  canal  varies  in  its  structure  and  extent 
according  to  food  habits.  Vegetarians  have  longer  ali- 
mentary canals  than  other  insects.  The  parts  of  the 
digestive  system  are:  a  mouth,  or  buccal  cavity;  a  slender 
esophagus;  a  thin- walled  crop;  a  glandular  stomach  from 
which  little  pouches,  or  caeca,  branch  out;  and  a  long 
slender  intestine.  At  the  junction  of  the  stomach  and 
intestine  the  slender  Malpighian  tubules  discharge  their 
excretions  into  the  alimentary  canal. 

The  respiration  of  insects  takes  place  through  the  tracheal 
system.  This  is  a  very  much  ramified  network  of  tubes 
which  carries  oxygen  to  all  parts  of  the  body.  The  air 
enters  the  tracheae  through  little  openings,  the  spiracles, 
along  the  sides  of  the  abdomen  and  thorax.  An  insect 
cannot  be  drowned  by  sticking  his  head  in  water;  the 
spiracles  must  be  covered  before  it  will  die.  Good  flyers 
have  their  tracheae  expanded  into  air  sacs,  which  make  the 
body  light,  but  insects. which  fly  little  have  no  such  ex- 
pansions (Fig.  36). 

Insects  usually  hatch  from  eggs.     As  they   grow  the 


68 


GENERAL  ZOOLOGY 


entire  skin  is  shed  at  intervals.  After  each  moult  the  body 
enlarges  rapidly  for  a  short  time,  but  when  the  exoskeleton 
once  hardens  there  is  no  more  growing  until  the  skin  is  shed 
again.  Insects  show  three  conditions  of  larval  life  and 


Fia.  36. — Insect  respiratory  systems.  A,  a.  flying  insect  with  exparisions.or 
air  sacs,  on  the  trachea!  tubes;  B,  an  insect  which  never  flies;  C,  a  portion  of  a 
tracheal  tube  enlarged. 

metamorphosis.  Ametabolous  insects  have  no  metamor- 
phosis. They  are  much  like  the  adult  in  form  when  hatched 
from  the  egg.  Heterometabolous  insects  hatch  as  nymphs, 
which  are  at  first  wingless,  but  have  gradually  larger  wings 


INSECTA  69 

after  each  moult,  until  the  adult  form  is  finally  attained. 
Holometabolous  insects  have  a  "  complete "  metamorphosis. 
They  hatch  from  the  egg  as  a  worm-like  larva,  which  feeds 
for  a  time;  then  goes  into  a  resting,  or  pupal,  stage  during 
which  no  food  is  taken.  The  larval  structures  are  com- 
pletely lost  as  the  pupa  is  formed.  When  the  pupa  moults, 
an  adult  insect,  or  imago,  comes  out. 

According  to  recent  usage,  insects  are  classified  into 
nineteen  orders.  These  will  now  be  taken  up  in  more  or 
less  detail. 

Order  1.  Aptera  (Fig.  37).— These  are  little  ametabol- 
ous,  wingless  insects  with  biting  mouth  parts.  Wingless- 


Fio.  37. — Insects  belonging  to  the  order  Aptera.     A,  a  collembolan;  B,  a- slicker, 
or  silver-fish;  C,  a  spring-tail. 

ness  in  this  case  represents  a  primitive  condition.  The 
little  slickers,  silver-fish,  or  fish  moths ;  the  spring-tails ;  and 
the  snow-fleas  are  examples.  Slickers  eat  the  glazing  on 
paper  and  starch  from  clothes.  They  are  frequently  seen 
in  dwellings  or  factories.  Spring-tails  are  expert  jumpers, 
being  able  to  hop  many  times  their  own  length.  The  snow- 
fleas  are  sometimes  so  abundant  as  to  give  the  snow  a 
characteristic  color. 

Order  2.  Ephemerida  (Fig.  38). — The  may-flies  live 
near  water.  They  are  heterometabolous  insects  with  biting 
mouth  parts.  Both  the  larval  and  adult  may-fly  are  easily 
recognized  by  the  long  setae  which  project  from  the  end  of 
the  abdomen.  There  are  three  of  these  on  the  larva,  and 
two  or  three  on  the  adult.  Eggs  laid  in  the  water  hatch  into 
nymphs  which  live  for  about  a  year,  feeding  on  aquatic  life. 


70 


GENERAL  ZOOLOGY 


A  nymph  when  mature  crawls  out  of  the  water,  splits  up 
the  back,  and  an  imago  emerges  from  the  old  skin.  The 
adult  may-fly  lives  only  a  day  or  two  and  keeps  fluttering 
along  the  shore,  looking  for  a  mate  or  laying  eggs.  Its 
mouth  parts  are  so  rudimentary  that  it  is  wholly  unable  to 
eat. 

Order  3.     Odonata  (Fig.  38). — This  order  includes  the 
dragon-flies  and  damsel-flies.     Both  are  largely  aquatic,  lay 


ODONATA  CPHCMfLRIDA        TRICHOPTEHA  PLECOPTERA 

FIG.  38. — Showing  the  immature  and  adult  stages  of  the  four  chief  orders  of 

aquatic  insects. 

their  eggs  in  the  water,  and  have  a  heterometabolous 
metamorphosis.  Damsel-fly  larvse  have  three  paddle-like 
gills  at  the  end  of  the  abdomen.  The  adults  are  sly  little 
insects  which  fold  their  gauzy  wings  up  over  their  backs 
when  at  rest.  Dragon-fly  larvse  have  three  short  spines 
at  the  end  of  the  abdomen.  They  swim  by  drawing  water 


INSECTA 


71 


in  through  the  anus  and  then  squirting  the  body  forward. 
The  adult  dragon-flies  rest  with  their  wings  stretched  out 
on  either  side  of  the  body.  They  are  expert  flyers  and  are 
of  benefit  to  man  on  account  of  the  great  numbers  of  mos- 
quitoes they  destroy.  A  dragon-fly  catches  all  its  prey 
on  the  wing.  Its  enormous  eyes  cover  the  whole  of  the 


FIG.  39. — Termite  nests.     At  the  right  is  a  section  of  a  tree  and  a  carton  nest; 
at  the  left  a  mound  nest  is  shown. 


head  (30,000  facets  are  present  in  one  species);  its  long 
light  wings  enable  it  to  fly  with  great  swiftness  in  any  direc- 
tion. The  larvsB  of  all  Odonata  have  a  peculiar  grasping 
pair  of  mandibles,  which  can  be  greatly  extended  to  capture 
food,  or  folded  up  like  a  hinge  beneath  the  head  when  not 
in  use. 

Order    4.     Plecoptera    (Fig.    38).— The    stone-flies    are 
heterometabolous,  aquatic  insects,  with  biting  mouth  parts. 


72  GENERAL  ZOOLOGY 

They  get  their  name  from  the  habit  the  larvae  have  of  hiding 
under  stones. 

Order  5.  Isoptera  (Fig.  39). — The  termites,  or  white 
ants,  have  four  leathery  wings  or  are  wingless,  possess  biting 
mouth  parts,  and  are  heterometabolous.  They  are  most 
abundant  in  tropical  countries  where  they  do  great  damage 
to  timber  and  wooden  structures.  They  never  come  out 
in  the  light,  but  secretly  dig  burrows  in  wood  until  there 
is  nothing  left  but  a  thin  outside  shell  which  finally  falls 
into  powder.  Many  species  of  termites  make  enormous 
nests  which  contain  thousands  of  individuals  living  as  a 
complex  spcial  community.  Division  of  labor  has  been 
carried  so  far  that  there  are  as  many  as  eight  different 
castes  in  some  species.  These  are  females,  males,  workers, 
soldiers,  etc.  The  worker  and  soldiers  castes  are  sexually 
immature  and  may  be  either  males  or  females.  The 
female  breaks  off  her  wings  after  mating  so  that  she  may 
not  stray  from  home.  When  mature  she  is  a  big  helpless 
egg-laying  sac,  and  must  even  be  fed  and  cleaned  by  her 
nurses.  She  grows  to  be  a  thousand  times  as  large  as  her 
mate. 

Orders  6,  7,  8,  9.  Corrodentia,  book-lice  and  bark-lice; 
Mallophaga,  biting  bird  lice;  Thysanoptera,  thrips;  Euplex- 
optera,  earwigs.  All  these  insects  have  biting  mouth  parts, 
and  are  heterometabolous,  except  the  Thysanoptera  which 
have  mouth  parts  somewhat  modified  for  sucking  and  a 
peculiar  metamorphosis  in  which  there  is  a  resting  stage. 
Different  species  of  thrips  are  pests  on  onion,  wheat,  grass, 
and  fruit. 

Order  10.  Orthoptera  (Fig.  40). — The  cockroaches, 
walking-sticks,  mantids,  grasshoppers,  locusts,  katydids, 
and  crickets  are  included  in  this  order.  These  insects  all 
have  biting  mouth  parts  and  are  heterometabolous.  Most 
of  them  have  four  wings;  the  anterior  pair  being  straight 
and  leathery;  the  posterior  pair,  membranous  and  folding 
like  a  fan.  There  are  six  important  families: 

Family  1.     Blattidce;  cockroaches.     These  insects  have 


INSECTA 


73 


the  legs  all  about  the  same  size  and  fitted  for  running. 
They  are  very  swift  and  for  the  most  part  nocturnal.  Their 
natural  home  is  in  rotten  stumps  or  under  loose  bark. 
Around  factories  and  apartment  houses  cockroaches  often 
cause  a  good  deal  of  trouble.  Sometimes  they  carry  dis- 
eases. A  finely  powdered  mixture  of  equal  parts  of  sugar 
and  borax  will  kill  them. 

Family  2.     Acrididce;  locusts,  or  "  short-horned "  grass- 
hoppers.    They  have  the  hind  legs  greatly  elongated  for 


FIG.  40. — Representatives  of  the  chief  families  of  Orthoptera.  From  left  to 
right  the  insects  are  as  follows:  tree  cricket  (Oecanthidce) ;  katydids,  female  and 
male  (Locustidce)  ;  mole-cricket,  field  cricket  (Gryllidoe) ;  grasshoppers^criduieE) ; 
mantis  and  egg-mass  (Mantidce) ;  cockroach  and  egg  case  (Blattidce) ;  walking- 
stick  (Phasmidce). 

leaping.  The  name  "  short-horn  "  refers  to  the  abbreviated 
antennae,  which  are  always  shorter  than  the  body.  A  rep- 
resentative of  this  family  will  be  discussed  in  detail  in  the 
next  two  chapters. 

Family  3.     Locustidce;  long-horned  grasshoppers,  katy- 
dids and  meadow  grasshoppers.     These  orthopterans  have 


72  GENERAL  ZOOLOGY 

They  get  their  name  from  the  habit  the  larvae  have  of  hiding 
under  stones. 

Order  5.  Isoptera  (Fig.  39). — The  termites,  or  white 
ants,  have  four  leathery  wings  or  are  wingless,  possess  biting 
mouth  parts,  and  are  heterometabolous.  They  are  most 
abundant  in  tropical  countries  where  they  do  great  damage 
to  timber  and  wooden  structures.  They  never  come  out 
in  the  light,  but  secretly  dig  burrows  in  wood  until  there 
is  nothing  left  but  a  thin  outside  shell  which  finally  falls 
into  powder.  Many  species  of  termites  make  enormous 
nests  which  contain  thousands  of  individuals  living  as  a 
complex  spcial  community.  Division  of  labor  has  been 
carried  so  far  that  there  are  as  many  as  eight  different 
castes  in  some  species.  These  are  females,  males,  workers, 
soldiers,  etc.  The  worker  and  soldiers  castes  are  sexually 
immature  and  may  be  either  males  or  females.  The 
female  breaks  off  her  wings  after  mating  so  that  she  may 
not  stray  from  home.  When  mature  she  is  a  big  helpless 
egg-laying  sac,  and  must  even  be  fed  and  cleaned  by  her 
nurses.  She  grows  to  be  a  thousand  times  as  large  as  her 
mate. 

Orders  6,  7,  8,  9.  Corrodentia,  book-lice  and  bark-lice; 
Mallophaga,  biting  bird  lice;  Thysanoptera,  thrips;  Euplex- 
optera,  earwigs.  All  these  insects  have  biting  mouth  parts, 
and  are  heterometabolous,  except  the  Thysanoptera  which 
have  mouth  parts  somewhat  modified  for  sucking  and  a 
peculiar  metamorphosis  in  which  there  is  a  resting  stage. 
Different  species  of  thrips  are  pests  on  onion,  wheat,  grass, 
and  fruit. 

Order  10.  Orthoptera  (Fig.  40). — The  cockroaches, 
walking-sticks,  mantids,  grasshoppers,  locusts,  katydids, 
and  crickets  are  included  in  this  order.  These  insects  all 
have  biting  mouth  parts  and  are  heterometabolous.  Most 
of  them  have  four  wings;  the  anterior  pair  being  straight 
and  leathery;  the  posterior  pair,  membranous  and  folding 
like  a  fan.  There  are  six  important  families: 

Family  1.     Blattidce;  cockroaches.     These  insects  have 


INSECTA  73 

the  legs  all  about  the  same  size  and  fitted  for  running. 
They  are  very  swift  and  for  the  most  part  nocturnal.  Their 
natural  home  is  in  rotten  stumps  or  under  loose  bark. 
Around  factories  and  apartment  houses  cockroaches  often 
cause  a  good  deal  of  trouble.  Sometimes  they  carry  dis- 
eases. A  finely  powdered  mixture  of  equal  parts  of  sugar 
and  borax  will  kill  them. 

Family  2.     Acrididce;  locusts,  or  "  short-horned "  grass- 
hoppers.    They  have  the  hind  legs  greatly  elongated  for 


FIG.  40. — Representatives  of  the  chief  families  of  Orthoptera.  From  left  to 
right  the  insects  are  as  follows:  tree  cricket  (Oecanthidai) ;  katydids,  female  and 
male  (Locustidce)  ;  mole-cricket,  field  cricket  (Gryllidce) ;  grasshoppers(.Acrwh'<ieE) ; 
mantis  and  egg-mass  (Mantidce) ;  cockroach  and  egg  case  (Blattidce) ;  walking- 
stick  (Phasmidas) . 

leaping.  The  name  "  short-horn  "  refers  to  the  abbreviated 
antennae,  which  are  always  shorter  than  the  body.  A  rep- 
resentative of  this  family  will  be  discussed  in  detail  in  the 
next  two  chapters. 

Family  3.     Locustidce;  long-horned  grasshoppers,  katy- 
dids and  meadow  grasshoppers.     These  orthopterans  have 


74  GENERAL  ZOOLOGY 

the  antennae  longer  than  the  body.  Many  show  protective 
resemblance  in  color  or  form  to  the  leaves  among  which  they 
live. 

Family  4.  Gryllidce;  crickets.  The  mole-crickets  have 
enormous  front  legs  and  spend  their  entire  lives  burrowing 
underground;  the  true  crickets  are  the  common  varieties 
whose  shrill  chirps  are  heard  in  the  fields  or  about  houses; 
the  tree  crickets  live  above  ground  in  vegetation  and  have 
very  harsh,  guttural  voices. 

Family  5.  Mantidce;  praying  mantids,  devil-horses.  In 
these  insects  the  first  pair  of  legs  form  prehensile  organs 
which  are  used  in  capturing  insects,  spiders,  etc.  Mantids 
live  in  bushes  or  trees  and  are  most  abundant  in  the 
warmer  parts  of  the  earth. 

Family  6.  Phasmidce;  walking-sticks.  These  curious 
orthopterans  have  small  legs  suited  for  walking.  Their 
form  resembles  the  twigs  on  which  they  live.  They  are 
very  slow  and  depend  upon  their  protective  resemblance 
to  escape  their  enemies.  If  disturbed  they  often  remain 
motionless  for  hours  at  a  time,  lest  they  betray  themselves. 
Their  food  consists  entirely  of  leaves. 

The  following  key*  will  separate  representatives  of  the 
Orthoptera  into  their  proper  families: 

1  (6)  Legs  equal  or  nearly  equal  in  size;  no  sound-producing  organs 

(Suborder,  Non-saltatoria),  2 

2  (3)  Body  short,  broad,  oval,  depressed;  head  almost  wholly  con- 

cealed beneath  the  thorax;  legs  compressed,  Family,  Blattidce. 

3  (2)  Body  elongate,   narrow;  head  free,  not  covered  by  thorax; 

legs  slender  and  not  all  compressed 4 

4  (5)  Front  pair  of  legs  fitted  for  grasping Family,  Mantidcs. 

5  (4)  Front  pair  of  legs  much  like  the  others. .  .  .Family,  Phasmidce. 

6  (1)  Hind  legs  fitted  for  leaping,  or  front  legs  greatly  enlarged  for 

burrowing;  organs  for  producing  sound  usually  present 

(Suborder,  Saltatoria),  7 

*  In  using  the  key  the  statement  after  a  particular  number  is  always 
to  be  compared  with  that  appearing  after  the  number  in  parenthesis,  which 
occurs  elsewhere  in  the  key.  For  example:  1  is  compared  with  6,  2  with  3; 
3  with  2,  etc. 


INSECTA  75 

7  (8)  Antennae  much  shorter  than  the  body;  calling  organs  of  male, 

when  present,  on  the  hind  femora  and  lower  border  of  first 
pair  of  wings;  organs  of  hearing,  when  present,  on  the  basal 
segment  of  abdomen;  ovipositor  short ....  Family,  Acrididce. 

8  (7)  Antennae  much  longer  than  body,  tapering;  calling  organs  of 

male,  when  present,  on  the  dorsal  surface  of  the  first  pair  of 
wings;  organs  of  hearing,  when  present,  on  the  tibiae  of  the 
first  pair  of  legs;  ovipositor  a  long  blade  or  needle 9 

9  (10)  First  pair  of  wings  slope  down  from  the  dorso-median  line; 

ovipositor  compressed  and  sword-like ....  Family,  Locustidce. 
10     (9)  First  pair  of  wings  flat  above,  the  sides  bent  abruptly  down- 
ward;  ovipositor   a   cylindrical,   straight,    or   slightly   curved 
needle,  usually  enlarged  at  the  tip Family,  Gryllidce. 


CHAPTER  VII 


PHYLUM  ARTHROPODA,  CLASS  INSECTA, 
ORDER    ORTHOPTERA 

THE  KINDS  OF  GRASSHOPPERS 

Grasshoppers,  or  short-horned  locusts  (Acrididce) ,  are 
numerous  the  world  over  wherever  appropriate  conditions 
are  found.  There  are  about  550  species  in  the  United 
States.  Most  of  these  do  not  range  throughout  the 
country — Indiana,  for  example,  has  only  about  65  species 
within  its  borders.  With  such  diversity  it  is  necessary 
to  have  some  means  for  dividing  grasshoppers  into  groups 
or  clans.  Zoologists  have  accordingly  separated  the  Family 
Acridida3  into  four  subfamilies,  and  these  may  be  dif- 
ferentiated by  the  following  key: 

1  (2)  Size  small;  first  segment  of  thorax  prolonged  backward  so  that 

it  reaches  to,  or  even  beyond,  the  posterior  end  of  the  abdomen; 
first   pair   of  wings  rudimentary.     The  grouse  locusts 

Subfamily,  Tettigiiice. 

2  (1)  Size  larger;  first  segment  of  thorax  never  extending  over  the 

abdomen;  first  pair  of  wings  usually  well  developed  as  tegmina, 
or  wing-covers 3 

3  (6)  No  spine  or  tubercle  on  the  sternum  between  the  front  pair  of 

legs 4 

4  (5)  Front  of  the  head,  or  face,  making  a  very  oblique  angle  with  the 

top  of  the  head;  wings  never  brightly  colored  and  without  a  black 
band Subfamily,  Tryaxalinos. 

5  (4)  Face  not  forming  a  sharp  angle  with  the  top  of  the  head;  second 

pair  of  wings  usually  with  some  yellow,  red  or  black 

Subfamily,  (Edipodince. 

6  (3)  A  distinct  conical  or  cylindrical  spine  on  the  front  margin  of  the 

sternum  between  the  first  pair  of  legs Subfamily,  Acridince. 

The  grouse-locusts   (Tettigince)  are   the   smallest  grass- 

76 


INSECTA  77 

hoppers.  They  pass  the  winter  as  mature  insects,  hiding 
beneath  stumps,  logs,  or  rubbish.  On  sunny  days,  they 
sometimes  venture  out  in  great  numbers,  even  in  mid- 
winter. On  the  first  warm  days  of  spring  they  can  be 
collected  by  hundreds  from  a  grass-covered  hillside  hav- 
ing a  southern  exposure,  or  from  boggy  places  along  the 
margins  of  lakes  and  streams.  They  feed  upon  vegetable 
mould,  humus,  lichens,  mosses,  tender  sprouts,  etc. 
Grouse-locusts  lay  eggs  underground  in  May,  and  the 
young  which  hatch  from  them  mature  by  autumn. 

The  TryaxalincB  for  the  most  part  frequent  the  borders 
of  marshes  and  damp  prairie  meadows.  Their  noiseless 
flight  and  quiet  colors  render  them  rather  inconspicuous. 
They  pass  the  winter  in  the  egg  stage,  mature  during  the 
summer,  and  die  in  the  autumn.  Many  species  have  the 
wings  short  and  imperfectly  developed. 

The  subfamily  (Edipodince  includes  many  locusts  that 
are  familiar  to  everyone.  In  all  these  insects  the  body  and 
outer  wings  are  dull  brown  or  gray.  Such  somber  colors 
blend  with  the  dusty  roads  and  bare  patches  which  they 
frequent.  When  in  flight,  however,  the  bright  colors  of 
the  inner  wings  are  displayed.  The  (Edipodince  sing  on 
the  wing — making  a  peculiar  crackling  sound.  They 
" crackle"  and  display  their  bright  colors  to  attract  mates, 
and  such  activities  make  them  rather  conspicuous.  The 
Carolina  Locust,  Dissostera  Carolina,  is  the  common  large 
species  along  roadways,  its  inner  wings  are  black  with  a 
broad  yellow  border.  A  few  of  the  grasshoppers  in  this 
family  pass  the  winter  as  half-grown  " nymphs,"  but  most 
species  winter  over  in  the  egg  stage. 

Most  of  our  common  small  grasshoppers  belong  to  the 
family  Acridince.  They  vary  much  in  size  and  general 
appearance.  Among  them  are  found  our  most  injurious- 
Orthoptera  for  they  commonly  feed  on  vegetation,  such  as 
farmers'  crops.  Most  species  are  dull  olive-brown,  or 
green,  frequently  with  stripes  or  spots,  and  none  have 
brightly  colored  wings.  Members  of  the  same  species  may 


78  GENERAL  ZOOLOGY 

vary  in  color  in  different  localities  or  at  different  seasons 
of  the  year.  The  genus  Melanopus  is  the  largest  one  in 
this  group.  Its  representatives  are  to  be  found  in  grass, 
which  is  their  chief  food.  The  males  of  the  Acridince 
usually  sing  when  at  rest  and  most  of  them  rarely  produce 
sounds  at  all.  In  order  to  get  a  concrete  idea  of  the  daily 
life  of  the  grasshopper,  we  will  now  consider  the  activities 
of  the  red-legged  locust. 

Wheeler  says:  "An  organism  is  a  complex,  definitely 
coordinated  and,  therefore,  individualized  system  of  activi- 
ties, which  are  primarily  directed  to  obtaining  and  assimi- 
lating substances  from  an  environment,  to  producing  other 
similar  systems,  known  as  offsprings,  and  to  protecting  the 
system  itself  and  usually  also  its  offspring  from  disturbances 
emanating  from  the  environment.  The  three  fundamental 
activities  enumerated  in  this  definition,  namely  nutrition, 
reproduction  and  protection,  seem  to  have  their  inception 
in  what  we  know,  from  exclusively  subjective  experience, 
as  feelings  of  hunger,  affection  and  fear  respectively." 
The  activities  of  the  red-legged  locust  may  properly  be 
considered,  therefore,  under  the  topics:  self-maintenance, 
self-protection,  and  race  preservation. 

You  may  wonder  why  anyone  should  study  a  grasshopper, 
or  why  the  writer  should  think  it  would  be  profitable  for 
you  to  do  so.  There  are  many  reasons  why  it  is  desirable. 
It  may  be  used  to  illustrate  many  general  zoological  facts 
and  principles,  with  particular  application  to  insect  prob- 
lems, and  you  may  perhaps  be  glad  after  all  that  we  did 
not  choose  elephants,  or  rattle  snakes  for  illustrative 
material.  If  we  are  going  to  look  into  the  daily  life  of 
the  grasshopper,  however — even  pry  into  his  pantry,  cup- 
board, sleeping  apartments,  courting  places,  and  other  most 
private  haunts — you  must  try  to  put  yourself  in  friend 
grasshopper's  place,  and  realize  to  the  extent  of  your  ability 
what -his  sensations,  wants,  and  difficulties  are.  If  we  know 
a  great  deal  about  one  animal,  we  will  be  prepared  to  make 
more  rapid  progress  with  others. 


INSECTA  79 

THE  RED-LEGGED  LOCUST,  OR  GRASSHOPPER 
Melanopus  femur-rubrum  DeGeer 

Self -maintenance. — The  grasshopper  eats  grass  or  almost 
any  other  vegetation.  Its  food  is,  therefore,  abundant 
and  easily  found.  If  hungry,  it  gives  a  few  hops  this  way 
and  that,  and  finally  lands  in  the  midst  of  plenty.  Once 
established  on  a  blade  of  grass,  its  jaws  start  swinging  side- 
ways, and  it  chews  away  until  satisfied.  The  strong 
mandibles,  or  jaws,  bite  off  the  grass.  The  other  mouth 
parts  help  manipulate  the  food  and  pass  it  on  into  the 
esophagus,  much  as  the  lips  and  tongue  help  the  teeth  in 
handling  our  food. 

The  grasshopper  is  well  provided  with  sense  organs 
which  may  help  it  in  finding  and  testing  its  food  (Fig.  41). 
The  antennae  are  supplied  with  tactile  bristles  for  feeling 
and  olfactory  pits  for  smelling.  The  sense  of  touch  is 
very  acute  but  smell  is  less  developed  than  in  insects  like 
flies,  which  find  their  food  by  its  odor.  In  addition  to  the 
olfactory  pits  of  the  antennae,  others  occur  on  the  body 
between  the  bases  of  the  legs.  On  some  of  the  mouth 
parts  there  are  gustatory  pits.  These  are  organs  of  taste 
which  test  the  palatability  of  the  food  before  it  enters  the 
mouth. 

There  are  five  eyes  on  the  head.  The  front  bears  three 
simple  eyes,  or  ocelli ;  two  near  the  bases  of  the  antennae  and 
one  near  the  center  of  the  forehead.  At  the  sides  of  the 
head  are  the  great  oval  compound  eyes,  made  up  of 
thousands  of  six-sided  ommatidia.  Each  ommatidium  is 
a  complete  organ  of  vision,  with  a  lens,  pigment,  a  sensitive 
nerve  ending,  and  nerve  fiber  connecting  with  the  optic 
ganglion  and  the  brain.  A  compound  eye  may  see  either 
a  mosaic  or  an  apposition  image.  In  the  first  type  there 
is  a  picture  of  the  surroundings  but  it  is  broken  up  into  a 
great  number  of  small  pieces,  each  of  which  is  perceived  by 
one  ommatidium.  In  an  apposition  image  the  view  in 
each  ommatidium  overlaps  those  adjacent  to  it  so  that 


82  GENERAL  ZOOLOGY 

Back  flow  of  the  blood  is  prevented  by  a  series  of  little 
valves  which  allow  only  movement  toward  the  head.  After 
the  blood  traverses  the  sinuses  (where  it  bathes  all  the 
internal  organs,  so  that  they  may  absorb  nourishment  from 
it  and  shed  excretions  into  it),  it  flows  into  the  pericardial 
chamber  and  enters  the  heart  through  several  pairs  of 
lateral  ostia.  If  more  food  is  digested  and  absorbed  than 
can  be  utilized,  it  is  converted  into  fatty  substances  and 
stored  in  the  two  elongated  "fat  bodies"  on  either  side 
of  the  heart. 

The  blood  of  a  grasshopper,  or.  of  any  tracheate  arthro- 
pod, does  not  carry  oxygen  or  carbon  dioxide  as  it  does  in 
most  animals.  The  trachea  form  separate  channels  for 
carrying  air  and  the  respiratory  system  is,  therefore,  en- 
tirely independent  of  the  circulatory  system.  This  is  an 
admirable  scheme,  for  each  system  is  free  to  carry  on  its 
own  work.  It  has  the  disadvantage,  or  course,  of  requiring 
two  separate  systems  of  conducting  tubes  in  all  parts  of  the 
body.  The  grasshopper's  tracheal  system  has  good-sized 
air-sacs  (Fig.  36),  which  indicate  that  it  is  well  fitted  for 
flight..  When  a  grasshopper  breathes  rapidly  the  abdomen 
is  rhythmically  shortened  and  lengthened.  By  such  move- 
ments air  is  forced  in  and  out  of  the  spiracles  on  the  sides 
of  the  thorax  and  abdomen.  A  tracheal  tube  will  not  col- 
lapse, for  the  walls  have  an  elastic  strand  in  them  which, 
though  not  continuous,  is  closely  coiled  like  a  wire  spring 
and  keeps  them  expanded.  The  tracheal  tubes  divide  into 
very  fine  branches  which  go  to  every  part  of  the  body. 
They  extend  to  the  tips  of  the  legs,  are  closely  wrapped 
about  all  the  internal  organs  and  even  penetrate  into  them. 
Oxygen  is  thus  always  at  hand  to  help  in  metabolism. 

The  excretions'  resulting  from  metabolic  processes  in  the 
grasshopper's  body  are  eliminated  through  Malpighian 
tubules  which  open  into  the  intestine  (midgut)  just  behind 
the  stomach.  These  tubules  are  closed  at  the  free  end. 
They  are  very  slender  and  swing  freely  about  in  all  direc- 
tions in  the  blood  sinuses.  Waste  products  are  absorbed 


IXSECTA  83 

through  their  thin  walls  and  discharged  into  the  intestine. 
These  waste  products,  which  result  from  the  breaking  down 
of  protein  compounds  (protoplasm),  take  the  form  of  urea 
or  of  uric  acid.  Uric  acid  is  a  crystalline  substance  not 
soluble  in  water.  Grasshoppers  that  live  in  very  dry  places, 
where  they  need  all  their  water,  have  uric  acid  as  their 
excretion  product.  Those  which  live  in  moist  places 
excrete  urea,  which  is  dissolved  in  water  and  eliminated  as 
urine. 


CHAPTER  VIII 

PHYLUM  ARTHROPODA,  CLASS  INSECTA,  ORDER 
ORTHOPTERA 

THE  RED-LEGGED  LOCUST  (Continued) 

Self -protection. — A  grasshopper  not  only  needs  to  secure 
food  but  must  also  escape  from  lurking  enemies  and  other 
dangers  if  it  is  to  be  successful.  Each  obstacle  must  be 
met  or  avoided  in  a  different  way.  The  grasshopper, 
therefore,  has  many  adaptations  for  enabling  it  to  slip  out 
of  difficulties  which  might,  in  the  absence  of  special  meas- 
ures, prove  its  undoing.  The  flight  of  red-legged  grass- 
hoppers is  comparatively  slow,  but  so  erratic  that  birds 
have  some  difficulty  in  catching  these  insects  on  the  wing. 
The  grasshopper's  method  of  escape  from  a  pursuing  enemy 
is  to  give  a  hop  or  make  a  short  flight,  then  rest  quietly  for 
a  time,  and,  if  again  threatened  with  immediate  danger, 
to  make  another  quick  change  of  locality.  Such  tactics 
are  continued  until  the  grasshopper  either  escapes  or  is 
captured.  An  expert  hunter,  like  a  turkey,  catches  the 
grasshopper  at  the  moment  it  alights,  before. there  is  time 
to  start  off  again.  Some  species  of  grasshoppers  are  so 
swift,  however,  that  they  easily  escape  capture  unless  taken 
unawares. 

When  a  grasshopper  rests  in  its  natural  haunts,  it  is 
difficult  to  see,  for  each  species  is  decked  out  to  match  its 
surroundings.  There  may  be  green  to  simulate  the  grass, 
dark  bands  to  resemble  the  shadows  among  the  vegetation, 
or  an  arrangement  of  colors  to  obliterate  the  outline  of  the 
grasshopper's  body  so  as  to  make  it  look  like  something  else. 
It  is,  of  course,  a  great  advantage  to  be  protectively  colored, 
and  the  grasshopper's  attitudes  make  its  resemblance  to 

84 


INSECTA 


85 


its  surroundings  more  complete.  When  at  rest  the  body  is 
held  parallel  with  a  blade  of  grass  so  as  to  be  easily  over- 
looked. Often  a  grasshopper  that  has  been  sitting  in  some 
careless  attitude  may  be  seen  to  change  its  position  when 
danger  approaches.  The  body  is  whirled  around  so  as  to 
lie  along  a  grass  blade  in  a  crouching  position — ready,  set 
for  a  jump  at  a  moment's  notice.  Our  red-legged  locust 


FIG.  42. — The  activities  of  grasshoppers.  A,  being  captured  by  a  robber 
fly;  B,  flying;  C,  dead  with  fungus  disease;  D,  caught  by  a  spider;  E,  laying  eggs 
F,  captured  by  a  digger  wasp;G,  H,  7,  nymphs. 

has  a  touch  of  yellow  and  brown  so  that  it  may  go  unseen 
among  the  dead  grass.  When  caught  napping  in  the  green 
foliage,  it  at  once  falls  head  over  heels  to  the  ground  where 
it  is  more  likely  to  escape  notice.  Some  relatives  of  'the 
grasshopper  show  extreme  cases  of  protective  resemblance. 
Certain  katydids,  and  "  leaf  -insects"  have  in  their  bodies 
reproduced  the  color  and  form  of  the  leaves  they  frequent, 


86  GENERAL  ZOOLOGY 

even  to  the  picturing  of  midribs,  veins,  and  other  fine 
details. 

Suppose,  though,  that  our  stupid  grasshopper's  simple 
wiles  are  of  no  avail,  and  that  it  is  caught  by  an  enemy. 
It  does  not  give  up  without  a  struggle — kicks,  and  strains 
to  the  best  of  its  powers  in  efforts  to  escape.  If  it  is  held 
by  only  one  leg  this  appendage  is  kicked  free  from  his  body 
and  gladly  left  in  the  enemy's  hands.  It  is  a  small  matter 
for  a  young  grasshopper  to  lose  a  leg;  somewhat  inconven- 
ient for  a  time,  perhaps,  but  no  serious  consequences  result. 
A  new  leg  is  formed  beneath  the  exoskeleton  and  becomes 
functional  at  the  next  moult.  An  adult  grasshopper, 
however,  cannot  grow  new  legs. 

Often  when  captured,  in  the  excitement  of  the  resulting 
struggle,  a  grasshopper  forms  great  quantities  of  the  brown 
"  molasses. "  or  crop  secretion,  which  drools  from  the  mouth. 
Such  secretory  activity  is  somewhat  comparable  to  the 
formation  of  "  cold  sweat "  on  the  skin  of  man  during. certain 
emotional  states.  In  the  present  instance,  the  molasses 
sometimes  serves  a  useful  purpose  by  making  the  grass- 
hopper distasteful  to  the  animal  holding  it  captive. 

Grasshoppers  are  comparatively  soft-bodied  and  of  good 
flavor.  They  are  hence  sought  for  food  by  a  great  number 
of  animals,  even  including  man  in  some  parts  of  the  world. 
They  are  not  even  safe  when  they  settle  down  to  rest  at 
the  close  of  a  day,  for  the  night-prowling  skunk  esteems 
grasshoppers  a  great  delicacy,  and  seeks  them  out  during 
the  night.  In  his  work  on  the  Orthoptera  of  Indiana, 
Blatchley  lists  forty-nine  birds  which  feed  upon  grasshoppers 
in  that  state.  Chief  among  these  are  hawks,  blackbirds, 
crows,  bluejays,  prairie  chickens,  mocking  birds,  and  blue- 
birds. Many  snakes  feed  very  largely  on  grasshoppers,  and 
their  sly  habits  make  them  hard  to  avoid.  Toads,  frogs, 
and  some  fish  also  prey  upon  these  insects  when  the  oppor- 
tunity offers.  Moles  and  shrews  eat  the  eggs  laid  by  grass- 
hoppers underground. 


INSECTA  87 

There  are  many  arthropods  which  use  grasshoppers  for 
food  (Fig.  42).  The  large  orb- weaving  spiders  spread  their 
snares  in  the  low  vegetation.  Small  mites  lurk  beneath 
leaves,  waiting  to  attach  themselves  to  some  unwary 
grasshopper  and  suck  his  blood.  Fierce  robber-flies  dart 
over  the  fields ;  ever  ready  to  pounce  down  and  carry  a 
grasshopper  to  some  retreat  to  be  devoured  at  leisure.  The 
wasps  and  beetles  are  also  important  enemies  among  the 
insects.  Some  wasps  store  their  nests  with  nothing  but 
grasshoppers. 

All  the  enemies  mentioned  up  to  this  time  seek  to  devour 
the  grasshopper,  piecemeal  or  in  toto.  We  have  now  to  con- 
sider certain  animals  which  invade  the  interior  of  the  body 
and  live  as  parasites. 

Often  on  a  hot  day  you  may  see  a  lazy  grasshopper  flap 
slowly  along,  changing  his  feeding  ground.  In  the  midst  of 
his  course  he  is  pursued  and  touched  for  an  instant  by  a 
swift  little  fly,  which  then  goes  about  its  business.  This  is 
a  tachina-fly,  and  it  needs  only  a  fraction  of  a  second  to 
poke  an  egg  between  the  segments  of  the  grasshopper's 
abdomen.  The  egg  hatches  into  a  maggot  which  straight- 
way begins  to  bore  his  way  into  and  through  the  interior 
of  the  grasshopper,  feeding  as  it  goes.  The  vital  organs 
are  left  until  the  last,  so  the  grasshopper  drags  out  a  miser- 
able existence  as  a  living  hotel  for  this  voracious  larva. 
Some  flesh-flies  also  lay  their  eggs  in  grasshoppers  when  they 
have  a  chance.  Certain  of  the  horsehair  worms  likewise 
live  for  a  part  of  their  life  cycles  in  these  abundant  insects. 

The  grasshopper,  like  man,  is  subject  to  certain  diseases 
which  make  it  sick  or  even  cause  death.  These  are,  in 
addition  to  the  large  parasites  already  mentioned,  mostly 
caused  by  bacteria  and  parasitic  fungi.  Such  pathogenic 
organisms  are  more  abundant,  and  hence  more  easily 
acquired,  during  damp  weather.  Wet  seasons  are,  there- 
fore, undesirable  for  the  grasshopper  but  good  for  the 
farmer.  Parasitic  plants,  such  as  bacteria,  are  able  to 
enter  the  body  more  readily  if  there  is  a  wound  on  the 


88  GENERAL  ZOOLOGY 

surface,  so  that  they  can  get  through  the  exoskeleton, 
which  usually  gives  protection  to  the  internal  organs. 

Perhaps  enough  has  been  said  to  show  that  the  grass- 
hopper pays  the  penalty  for  being  abundant  and,  as  in- 
sects go,  to  a  reasonable  degree  successful.  It  is  beset  by 
a  host  of  enemies  which  in  various  ways  try  to  make  an 
end  of  it.  If  we  have  "put  ourselves  in  the  grasshopper's 
place,"  we  may  now  feel  a  certain  sympathy  for  the 
creature — but  there  is  another  side  to  its  case.  If  grass- 
hoppers have  a  good  season,  with  abundant  food,  few 
enemies  and  favorable  meterological  conditions,  it  will 
be  a  bad  year  for  crops.  In  many  parts  of  the  United  States 
in  fact,  the  grasshopper  is  successful  enough  to  be  a  con- 
tinual menace,  and  in  some  seasons  becomes  a  veritable 
calamity.  This  has  led  man  to  devise  means  for  his  own 
protection,  and  these  have  been  developed  along  four  lines: 
(1)  traps,  (2)  poisonous  sprays,  (3)  inoculation  with  diseases 
or  the  introduction  of  natural  enemies,  and  (4)  poison 
baits. 

On  certain  crops,  grasshoppers  may  be  killed  by  using 
sprays  (arsenate  of  lead,  Paris  green,  kerosene,  etc.). 
Their  method  of  feeding  renders  them  readily  susceptible 
to  poisons  sprayed  on  the  outside  of  plants.  Oils,  like 
kerosene,  enter  the  tracheal  tubes  and  kill  in  that  way. 
Such  methods  are  not,  however,  readily  practised  on  a 
comprehensive  scale.  In  large  fields  trapping  is  often 
resorted  to  as  a  means  of  control.  One  method  is  to  set 
large  cans  of  water  and  kerosene  below  the  surface  of  the 
ground  in  places  where  the  insects  are  most  abundant. 
In  hopping  about  the  grasshoppers  fall  into  the  traps  and 
are  killed.  The  hopperdozer  (Fig.  43)  is  a  movable  trap 
which  is  driven  over  infected  fields.  The  grasshoppers 
hop  or  fly  as  it  comes  near,  fall  into  the  kerosene  pan,  and 
die. 

In  the  way  of  living  enemies,  a  flock  of  turkeys  will  do 
as  efficient  work  as  any  remedy.  Other  measures  consist 
in  the  introduction  of  parasitic  flies,  or  in  the  liberation 


INSECTA 


89 


of  grasshoppers  already  infected  with  contagious  diseases. 
Several  years  ago  a  plague  of  locusts  was  successfully  fought 
in  Argentina  by  liberating  a  fungous  disease.  More 
recently  in  New  Mexico  a  fly,  Sarcophaga  kellyi,  "was 
found  to  be  by  far  the  most  important  factor  in  control." 
Some  wild  birds  (horned  lark,  meadow  lark,  sparrow  hawk, 
kildeer,  quail)  also  served  as  loyal  allies  of  man  on  this 
occasion. 

The  great  "plagues"  of  grasshoppers  in  the  United  States 
have  in  most  cases  been  due  to  the  ravages  of  the  migratory 
locust,  Melanopus  spretus,  which  travels  in  enormous  flocks 


FIG.  43. — A   hopperdozer.     Kerosene   is   placed   in   the   trough    which   is   then 
dragged  over  a  field. 

through  the  high  prairies  of  Western  Kansas,  Nebraska,  and 
Eastern  Colorado.  In  the  plague  of  1913,  another  species, 
Dissostera  longipennis,  was  so  abundant  that  it  repeatedly 
stopped  trains  by  massing  on  the  railroad  tracks.  It 
rendered  an  area  of  500  square  miles  unfit  for  grazing. 
Grasshoppers  not  only  harm  man  by  feeding  on  crops, 
but  do  injury  by  spreading  the  spores  of  various  plant 
diseases. 

If  a  grasshopper  successfully  escapes  all  its  enemies,  it 
still  cannot  rest  in  idle  ease,  but  must  make  continual 
adjustments  to  the  changing  conditions  in  its  surroundings. 
During  rains  the  big  head  and  roof-like  wings  protect  it 


90  GENERAL  ZOOLOGY 

somewhat,  but  wet  weather  is  bad  for  grasshoppers  for 
then  diseases  multiply  and  other  difficulties  arise.  A  few 
grasshoppers  pass  the  winter  as  adults,  or  half-grown 
nymphs.  The  red-legged  locust,  however,  gives  up  the 
struggle  in  the  autumn,  but  before  dying  lays  a  mass  of 
eggs  which  hatch  in  the  following  spring.  This  brings 
us  to  the  last  aspect  of  the  grasshopper's  life — race 
preservation. 

Race  Preservation. — Grasshoppers  are  of  separate  sexes, 
males  and  females.  A  female  is  readily  distinguished  ex- 
ternally by  the  ovipositor,  or  " egg-layer,"  at  the  tip  of  the 
abdomen.  This  is  composed  of  two  short  curved  horny 
plates,  which  cover  the  opening  for  the  genital  organs. 
The  internal  reproductive  organs  in  a  female  grasshopper's 
abdomen  consist  of  a  series  of  long  tubes  modified  in  dif- 
ferent regions  to  form  a  pair  of  ovaries,  oviducts,  shell 
glands,  and  a  seminal  receptacle.  The  ovaries  are  glands 
which  give  off  living  egg  cells.  Each  consists  of  a  number 
of  little  tubes  which  connect  with  the  oviduct.  A  male  has 
in  his  abdomen  a  pair  of  testes  which  produce  sperm  cells, 
and  two  long  twisted  vasa  deferentia,  or  sperm  ducts. 
Extending  at  the  posterior  end  are  a  pair  of  cerci  for 
introducing  sperm  into  the  female. 

If  red-legged  locusts  are  to  eat  grass  year  after  year  there 
must  be  a  continual  series  of  new  generations.  These  are 
produced  in  only  one  way — from  eggs.  A  female  grass- 
hopper can  grow  eggs  by  herself  and  even  lay  them  in  some 
cases,  but  the  eggs  will  not  hatch  out  young  grasshoppers 
unless  they  have  been  fertilized.  "  Fertilization  "  consists 
primarily  in  the  union  of  a  sperm  cell  nucleus  with  that  in 
an  egg  cell.  In  order  to  insure  fertilization  the  two  sexes 
must  be  brought  together. 

The  coming  together  of  the  male  and  female  grasshopper 
is  not  altogether  a  matter  of  accident.  The  male  takes  the 
initiative.  If  you  sit  quietly  by  a  dusty  road  on  a  summer 
day,  you  cannot  fail  to  observe  the  Carolina  locusts.  The 
males  at  intervals  rise  from  the  ground,  flap  their  wings 


INSECTA  91 

for  a  minute,  and  then  settle  down  again.  The  "flapping" 
is  to  notify  females  that  an  amorous  male  is  abroad.  It 
displays  the  yellow  and  black  of  the  under  wings  and  the 
sound  produced  has  a  certain  pitch  which  is  characteristic 
for  each  species  in  the  family  to  which  the  Carolina  locust 
belongs.  If  there  is  an  unfertilized  female  near,  she  comes 
to  the  male  and  he  pokes  a  little  packet  'of  sperm  into 
her  seminal  receptacle  with  his  cerci.  She  is  now  pre- 
pared to  lay  fertile  eggs.  The  red-legged  locust  does  not 
make  such  a  splendid  display  as  its  larger  relative,  just 
described.  However,  it  sits  in  the  grass  and  sings  a  feeble 
song  which  is  easily  recognized  by  females  of  its  own 
species. 

The  bright  "  recognition "  colors,  the  sound-producing 
organs,  and  ears,  are  of  little  or  no  value  in  procuring  food 
or  for  self-protection,  but  are  adaptations  for  mating!  So 
grasshoppers  use  many  means  to  insure  fertile  eggs,  and  the 
continuance  of  the  race. 

After  a  female  grasshopper  has  been  " fertilized"  the 
sperm  cells  move  to  the  eggs  and  enter  them,  the  egg  and 
sperm  nuclei  fusing  in  the  cytoplasm  of  the  egg  cell.  Thus 
a  single  cell  is  formed  by  the  union  of  the  egg  and  sperm 
cells.  This  new  cell  is  the  zygote.  It  has  all  the  character- 
istics of  youth,  as  described  in  Chapter  III,  and  is  the 
beginning  of  a  new  individual. 

Soon  after  the  zygote  has  been  formed  it  begins  to  seg- 
ment, or  undergo  mitotic  cell  division  (Fig.  25).  First  the 
nucleus  divides  several  times  and  the  new  nuclei  arrange 
themselves  near  the  outside  of  the  egg ;  the  cytoplasm  about 
each  nucleus  becomes  separate  from  the  rest  and  a  number 
of  little  cells  are  thus  formed  just  beneath  the  egg-shell 
(Fig.  44,  A,  B,  C).  Development  takes  place  on  the  out- 
side of  the  egg  because  the  yolk,  or  food  material  fills  the 
interior.  As  the  cells  continue  to  multiply  a  little  embryo 
is  soon  formed  on  one  side  of  the  "egg"  (D),  and  this  later 
is  enveloped  by  two  protective  membranes  which  are  known 
as  the  amnion  and  serosa  (E,  F).  Finally  the  yolk  is  all 


92 


GENERAL  ZOOLOGY 


FIG.  44. — Development  of  a  grasshopper's  egg;  A,  an  unsegmented  egg  con- 
taining a  single  nucleus  near  the  center;  B,  an  egg  in  which  the  original  nucleus 
has  divided  many  times  and  the  daughter  nuclei  are  arranged  around  the  out- 
side of  the  egg;  C,  a  later  stage  in  which  the  yolk  material  is  completely  enclosed 
by  cells;  D,  the  thickening  on  one  side  indicates  the  beginning  of  an  embryo; 
E,  embryo  partly  overgrown  by  amnion;  E',  cross  section  of  same;  F,  amnion 
and  serosa  completely  covering  embryo;  F',  cross  section  of  same;G,  insect  with 
appendages  forming. 


INSECTA 


93 


used  up  and 'the  young  grasshopper  is  ready  to  hatch  (G). 
Soon  after  fertilization  the  female  grasshopper  drills  a 
hole  in  the  ground  with  her  abdomen  and  deposits  thirty 
or  forty  eggs,  which  remain  covered  until  spring.  Most  of 
the  development  within  the  egg-shell  therefore  takes  place 
after  the  eggs  are  laid.  In  spring,  when  the  ground  is 
warm  enough,  the  egg-shells  break  and  little  wingless 
nymphs  work  their  way  to  the  surface.  The  young  grass- 
hoppers begin  to  feed  at  once,  but  cannot  grow  on  account 
of  their  hard  exoskeletons.  This  difficulty  is  overcome  by 
a  periodical  casting  of  the  outer  skin  (Fig.  45).  After  such 
a  moult  a  grasshopper  is  soft  for  a  short  time  and  grows 


D  £ 

FIG.  45. — A  grasshopper  nymph  shedding  its  skin.     (After  Packard.)     In  E  the 
adult  which  emerged  is  drying  its  wings;  in  F,  it  is  ready  to  fly  away. 

very  rapidly.  Then  the  exoskeleton  hardens  again  and 
there  can  be  no  more  growth  until  the  next  moult.  After 
the  first  moult  a  tiny  pair  of  wings  appear.  These  increase 
in  size  each  time  the  skin  is  shed,  but  do  not  become  func- 
tional until  the  last  moult,  when  complete  maturity  is 
attained.  After  becoming  mature  the  grasshopper  seeks  a 
mate,  starts  off  a  new  generation,  and  dies.  Thus  the 
cycle  goes  round.  The  grasshopper  must  strain  every 
faculty  to  keep  up  its  race.  Its  arduous  struggle  has  but 
one  end — to  produce  offspring  and  die. 

But  is  this  all?  The  characteristics  of  animals  are 
handed  down  with  great  fidelity  generation  after  genera- 
tion. Heredity  is  jealous  of  any  change.  But  as  the  ages 


94  GENERAL  ZOOLOGY 

go  by  a  race  improves  a  little  from  time  to  time,  if  it 
struggles.  This  is  an  unvarying  biological  law — struggle 
and  trying  bring  improvement;  lack  of  struggling  and  trying 
bring  retrogression  and  degeneracy.  Has  the  grasshopper 
improved?  If  we  compare  a  mature  grasshopper  with  its 
remote  ancestors  (Figs.  27,  28)  there  is  no  gainsaying  that  it 
has  gone  ahead.  It  has  lost  the  awkward  arrangement  of 
legs  on  each  segment  of  the  body  and  condensed  them  on 
the  three  thoracic  segments  so  that  it  can  turn  quickly  and 
easily.  The  legs  are  specialized  for  certain  purposes ;  wings 
have  developed,  improvements  have  been  made  in  methods 
of  breathing,  eating,  etc.  The  ancestral  grasshoppers  did 
their  work  better  from  day  to  day,  and  the  race  has 
improved! 

Now,  how  much  has  our  grasshopper  attained?  How 
much  ability  does  it  display  in  adjusting  itself  to  its 
surroundings?  Its  structure  is  highly  specialized  and 
admirably  adjusted  to  the  life  it  lives,  but  its  mind  is  of 
rather  low  order.  Most  of  its  activities  are  reflexes  or 
tropisms — i.e.,  they  are  repeated  each  time  an  individual  is 
subjected  to  the  same  set  of  external  conditions.  A  grass- 
hopper eats  when  it  is  hungry,  rests  when  satiated,  breeds 
when  ripe,  and  carries  on  many  other  routine  activities — 
but  does  it  reason?  We  must  answer,  no,  for  the  most  part. 
A  grasshopper  can  make  simple  adjustments  in  its  behavior 
to  adapt  itself  to  minor  changes  in  its  environment,  but  its 
reactions  are  usually  stereotyped.  If  you  put  one  in  a 
tumbler,  it  bumps  its  head  over  and  over  again  in  efforts 
to  escape  and  there  is  no  "intelligent"  effort  to  seek  an 
exit.  Yet,  notwithstanding  the  grasshopper's  present  lowly 
mental  state,  we  do  not  know  what  it  may  attain  in  the 
future.  In  its  discrimination  of  bright  colors  and  specific 
songs  we  can  see  the  first  glimmerings  of  appreciation, 
satisfaction,  pain,  and  pleasure. 


CHAPTER  IX 

PHYLUM  ARTHROPODA,  CLASS  INSECTA 

(Continued) 

Order  11.  Hemiptera. — This  order  includes  the  "bugs" 
proper,  and  no  other  insects  should  be  called  by  that  name. 
Bugs  are  heterometabolous,  have  sucking  mouth  parts,  and 
may  possess  two  pair  of  wings  or  be  without  any.  There 
are  three  suborders: 

Suborder  1.  Parasita. — This  group  comprises  the  lice, 
which  are  wingless  and  parasitic.  Lice  do  a  small  amount 


A  B  C 

FIG.  46. — Lice.     A,  eggs  of  head  louse  attached  to  a  hair;  B,  head  louse;  C, 

body  louse. 

of  damage  by  making  punctures  and  sucking  blood; 
but  are  dreaded  chiefly  because  they  may  inoculate  man 
with  the  germs  of  certain  diseases  when  they  pierce  the  skin. 
In  most  parts  of  the  world  there  are  three  species  which 
attack  man  and  many  more  which  prey  upon  other  animals. 
Pediculus  humanus  is  the  head  louse  (Fig.  46).  It  is 
found  the  world  over  and  its  color  varies  somewhat  accord- 

95 


96  GENERAL  ZOOLOGY 

ing  to  the  race  upon  which  it  is  found.  It  is  very  common 
in  many  tropical  countries  where  the  natives  may  often 
be  seen  searching  through  each  others  heads  for  the  para- 
sites. The  female  head  louse  lays  about  fifty  eggs,  attached 
to  hairs.  Pediculus  corporis,  the  body  louse,  is  larger  than 
the  head  louse.  It  is  gray  in  color  and  hence  is  known  in 
some  localities  as  the  "gray-back."  This  louse  causes 
much  irritation,  crawling  over  the  body  and  inflicting 
painful  bites.  The  eggs  are  laid  in  the  seams  of  garments. 
The  young  lice  or  the  adults  may  be  active  in  discarded 
clothing.  The  crab-louse,  Phthirius  inguinalis,  is  found 
among  the  hairs  of  the  pubic  region,  eyelashes,  eyebrows, 
and  sometimes  on  other  parts  of  the  body  where  it  causes 
irritation,  eruption,  and  discoloration  of  the  skin.  It 
attaches  its  eggs  to  hairs. 

Though  fleas  are  the  chief  carriers  of  the  bubonic  plague, 
lice  may  transmit  the  disease  and  have  also  been  suspected 
of  carrying  leprosy.  Lice  are  of  chief  interest,  however,  as 
the  proven  carriers  of  typhus,  a  fatal  febrile  infection  of 
unknown  cause.  Body  lice  of  course  thrive  most  when 
people  are  filthy.  The  terrible  outbreaks  of  typhus  fever 
in  Mexico  and  Serbia,  following  the  advent  of  war,  were  due 
to  poverty  and  uncleanliness  among  a  rather  ignorant 
population.  The  chief  means  used  in  fighting  these  great 
epidemics  were  simple  sanitary  measures  which  would  kill 
or  prevent  the  breeding  of  lice — i.e.,  boiling  clothes  regularly, 
fumigation  of  public  conveyances,  and  the  use  of  blue 
ointment,  paraffin  oil,  or  kerosene  on  the  surface  of  the  body. 

Suborder  2.  Homoptera  (Fig.  47). — Hemipterous  in- 
sects with  four  similar  membranous  wings;  the  mouth 
parts  form  a  jointed  beak  which  arises  from  posterior  ventral 
part  of  the  head.  Many  insects  of  great  economic  impor- 
tance belong  here — scale  insects,  plant  lice,  cicadas,  tree- 
hoppers,  spittle  insects,  etc.  All  these  obtain  their  food 
by  burying  their  beaks  in  plants  and  sucking  the  sap.  The 
three  following  families  are  of  particular  importance: 

Aphidce,  plant  lice  or  aphids.     These  little  insects  are 


INSECTA 


97 


all  of  small  size,  and  often  occur  in  great  numbers  on 
plants.  In  spring  and  during  the  summer  certain  females, 
the  "stem  mothers,"  bring  forth  living  young  which  have 
developed  from  unfertilized  eggs.  In  the  autumn  male 
individuals  appear,  and  fertilized  eggs  are  laid  which  pass 
through  the  winter.  Several  different  kinds  of  individuals 


FIG.  47. — Homoptcra.  At  the  left  are  plant-lice,  or  aphids,  one  being  de- 
voured by  a  lady-bird  beetle  (not  Homoptera) ;  above  are  leaf-hoppers  and 
"brownie  bugs,"  or  tree-hoppers;  on  the  tree  trunk  is  a  cicada,  or  seventeen- 
year  locust,  which  has  just  emerged  from  its  nymphal  skin,  and  two  species  of 
scale  insects. 

are  often  present  in  the  same  species:  winged,  wingless, 
woolly,  smooth,  large,  small,  etc.  The  grape-phylloxera 
is  a  destructive  species  which  perforates  the  roots  of  plants 
and  starts  diseases;  green-fly  on  wheat,  and  the  woolly 
apple  aphis  are  also  important. 

Coccidce,  scale  insects.     These  are  particularly  destruc- 


98  GENERAL  ZOOLOGY 

tive  to  trees  and  fruit.  They  are  always  of  minute  size 
but  may  be  very  abundant.  The  male  is  winged  and 
moves  about  freely.  The  female  buries  her  beak  in  a 
plant  when  young  and  never  moves  again.  She  has  no 
wings,  and  her  body  is  covered  by  a  secreted  "  scale." 
She  lays  eggs  which  hatch  beneath  the  scale.  The  young 
females  settle  down  near  their  mother,  hence  scale  insects 
spread  rather  slowly.  The  most  important  species  eco- 
nomically are  the  San  Jose  scale,  introduced  originally  into 
California  but  now  found  in  most  parts  of  the  United  States, 
the  cottony  cushion  scale,  and  the  oyster  scale. 

Cicadidce,  the  cicadas.  The  seventeen-year  " locust"  is 
the  best  known  representative  of  this  family.  The  adults 
sing  discordant  notes  in  trees  during  late  summer  and,  after 
mating,  lay  eggs  in  the  twigs  of  trees.  The  larvae  hatch 
in  about  six  weeks,  drop  to  the  ground,  and  live  under- 
ground for  nearly  seventeen  years  as  ugly  nymphs.  The 
nymph  burrows  about  in  the  soil,  feeding  on  roots  and 
humus.  In  the  summer  of  the  seventeenth  year  it  crawls 
out  of  the  earth,  climbs  upon  some  plant,  and  an  adult 
cicada  emerges  from  its  skin  (Fig.  47). 

Other  insects  belonging  to  this  suborder  are  the  tree- 
hoppers,  leaf-hoppers,  lantern-flies,  white-flies,  spittle  in- 
sects, etc.  The  tree-hoppers  and  leaf-hoppers  have  peculiar 
triangular  or  pointed  heads.  The  spittle  insects  are  found 
within  a  mass  of  froth  on  low  vegetation. 

Suborder  3.  Heteroptera,  the  True  Bugs  (Fig.  48).— 
In  this  order  the  fore  and  hind  wings  differ;  the  former 
being  thick  in  front  and  thin  behind;  the  latter,  entirely 
membranous.  The  peculiar  fore-wings  give  the  name 
(Hemiptera — half  wings)  to  this  whole  order  of  insects, 
though  they  are  of  course  not  characteristic  of  the  first 
two  suborders.  Several  insects  of  great  economic  impor- 
tance belong  to  this  group,  and  many  are  common  "  water- 
bugs." 

Among  the  aquatic  Heteroptera  are  many  with  interest- 
ing habits.  All  breathe  air  and  must  therefore  have  special 


INSECTA  99 

means  for  respiration.  Corixa  is  the  water-boatman 
which  has  the  hind  legs  flattened  to  form  two  great  oars. 
When  swimming  under  water  it  carries  air  between  the 
fine  hairs  which  cover  the  body  and  beneath  the  wings, 
hence  is  able  to  stay  submerged  for  a  long  time.  Noto- 
necta,  the  back-swimmer,  is  something  like  Corixa,  but 
usually  swims  with  its  ventral  surface  up.  The  water- 
scorpions  (Nepa,  Ranatra) 
have  long  slender  breath- 
ing tubes  for  taking  air 
from  the  surface  of  the 
water;  Nepa  is  flat,  Ra- 
natra is  long  and  slender. 
Belostoma  is  the  giant 
water-bug  or 
light"  bug,  which  often 
flies  over  the  land  at  night. 
Its  small  relative,  Zaitha, 


FIG.  48. — Heteroptera.  From  left  to  right  the  insects  are  the  water-boatman 
(Corixa) ;  back-swimmer  (Notonecla) ;  water-stridcrs  (Gerris) ;  a  water-scorpion 
(Nepa)  catching  a  minnow;  a  male  water-bug  (Zaitha)  carrying  eggs  on  his  back; 
chinch-bugs;  flower- bug;  assassin-bug;  squash-bug. 

is  of  interest  on  account  of  its  peculiar  breeding  habits. 
The  female  sticks  her  eggs  on  the  back  of  a  male,  and  he 
is  obliged  to  carry  them  around  till  they  hatch.  The 
marsh-treaders  and  water-striders  (Gerris,  etc.)  run  over 
the  surface  of  the  water  with  great  skill.  They  are  kept 
from  sinking  through  the  surface  film  by  the  little  hairs  on 
the  legs  which  cannot  be  wet.  All  the  aquatic  bugs  are 


100  GENERAL  ZOOLOGY 

predaceous,  feeding  on  insects,  worms,  small  fish,  and  tad- 
poles. 

The  terrestrial  bugs  in  this  suborder  are  common  every- 
where. The  flower-bugs  are  small  in  size  but  often  very 
numerous  on  plants,  where  they  lurk  to  catch  minute 
insects.  The  large  assassin-bugs  feed  mostly  on  insects. 
They  pierce  them  with  their  strong  beaks  and  suck  the 
juices.  Some  species,  such  as  the  one  most  often  known 
as  a  "kissing-bug,"  Reduvius  personatus,  are  able  to  inflict 
painful  bites  on  man.  The  ambush  bugs  are  colored  like 
flowers,  and  this  enables  them  to  capture  small  insects  which 
come  to  the  blossoms  they  frequent.  There  are  over  250 
species  of  leaf  bugs  (Capsidce)  in  the  United  States,  and  many 
are  garden  pests,  though  some  are  predaceous.  The  squash- 
bug,  Anasa  trislis,  is  well  hated  by  every 
gardener,  being  a  persistent  enemy  of 
squashes.  The  chinch-bug,  Blissus  leucop- 
terus,  occurs  throughout  the  United  States 
and  in  parts  of  Canada.  It  is  particularly 
abundant  in  dry  seasons.  The  annual  loss 
in  the  United  States  due  to  this  insect  has 
been  estimated  at  $20,000,000.  The  stink- 

FIG.  40.— The         ,  .,  .       j    ,         , ,     .         , 

bed-bug.  bugs  are  easily  recognized  by  their  char- 
acteristic odor.  Some  species  feed  on 
plants,  others  on  insects. 

The  bed-bug,  Cimex  lectularius  (Fig.  49)  is  parasitic  and 
has  the  wings  represented  by  very  small  scales.  It  lays  its 
eggs  in  cracks,  nail  holes,  mattresses,  under  wall  paper, 
and  in  other  suitable  places.  It  is  entirely  nocturnal, 
feeding  while  its  victims  slumber.  Killing  a  bed-bug  by 
hand  is  no  easy  matter.  The  body  is  so  flat  that  it  can 
stand  considerable  pressure  without  being  injured.  In 
India  and  certain  other  warm  countries  the  bed-bug  car- 
ries an  important  human  disease  known  as  kala-azar. 

Order  12.  Neuroptera  (Fig.  50). — This  is  a  small  order 
which,  however,  contains  some  very  interesting  insects. 
All  have  four  membranous  wings  with  many  "veins," 


INSECTS  -  •«,•,«,'„  -    .101 


biting  mouth  parts,  and  a  complete  metamorphosis  (holo- 
metabolous).  The  ant-lions'  are  the  larvae  of  a  neurop- 
terous  insect,  Myrmelion.  They  live  in  little  burrows 
which  open  at  the  bottom  of  conical  pits.  When  a  small 
insect  starts  to  slide  down  the  side  of  the  pit,  they  throw 
sand  at  it.  The  victim  finally  reaches  the  formidable  jaws 
waiting  at  the  bottom  of  the  pit.  The  aphis-lion  hatches 


FIG.  50. — Neuroptera.  At  the  left  is  the  lace  wing  with  its  eggs  and  nymph 
(aphis-lion) ;  in  the  center  the  ant-lion  and  the  insect  which  emerges  from  it  are 
shown;  at  the  right  is  the  dobson-fly  and  its  larva. 

from  curious  little  stalked  eggs,  and  spends  its  larval  life 
searching  over  plants  for  aphids,  which  constitute  its  chief 
food.  It  thus  does  much  good  to  man.  The  adult  of  the 
aphis-lion  is  the  beautiful  little  lace-winged  fly,  Chrysopha. 
The  dobson-fly,  or  hellgrammite,  Corydalis  cornuta,  is  a 
large  insect  (four  inches  across  with  spread  wings)  with 


102  GENERAL  ZOOLOGY 

vicious-looking  jaws.  The  larva  is  known  by  many  names 
(dobson,  crawler,  amly,  conniption  bug,  clipper,  water 
grampus,  goggle  goy,  bogart,  helldevil,  flip-flop,  etc.),  and 
is  often  used  by  fishermen  for  bait.  It  is  an  inch  or  more 
in  length,  has  three  pairs  of  legs  and  a  fringe  of  slender  gills 
along  either  side  of  the  abdomen. 

Order  13.  Mecoptera. — The  scorpion-flies  possess  wings 
similar  to  the  Neuroptera;  the  head  is  prolonged  ventrally 
into  a  long  beak  which  has  biting  jaws  at  its  tip;  metamor- 
phosis is  holometabolous.  The  common  name  refers  to 
the  structure  present  in  some  species  at  the  end  of  the  male 
abdomen  which  somewhat  resembles  the  sting  of  a  scorpion. 

Order  14.  Trichoptera,  Caddis-flies  (Fig.  38).— The 
larvae  of  the  insects  in  this  order  are  the  caddis-"  worms " 
which  make  little  tubes  or  cases  to  protect  the  body.  The 
tubes  are  usually  carried  about  with  the  larvae,  but  are 
sometimes  fastened  to  rocks  or  stones.  They  are  character- 
istic for  each  species,  in  regard  to  both  materials  and  archi- 
tecture. Some  species  make  their  cases  entirely  of  snail 
shells,  others  use  sticks,  leaves,  sand,  etc.  Adult  caddis- 
flies  have  four  membranous  wings,  covered  with  little  hairs. 
They  live  only  a  short  time  (long  enough  to  mate  and  lay 
eggs)  but  often  occur  in  great  numbers  near  bodies  of 
water  where  they  passed  their  larval  life.  The  mouth  parts 
of  caddis-flies  are  rudimentary,  as  in  may-flies. 

Order  15.  Lepidoptera  (Fig.  51). — Here  belong  the 
butterflies,  skippers,  and  moths.  These  insects  have  four 
wings  covered  with  scales;  the  mouth  parts  form  a  long 
sucking  tube;  all  are  holometabolous. 

The  larvae  of  lepidopterous  insects  are  caterpillars,  which 
often  do  damage  by  gnawing  vegetation.  The  adults  feed 
largely  on  nectar,  sucked  out  of  flowers  by  means  of  the 
long  proboscis.  When  not  in  use  the  proboscis  is  coiled  up 
beneath  the  head.  There  are  over  7000  species  of  Lepidop- 
tera in  the  United  States,  and  many  are  of  great  economic 
importance.  This  vast  array  is  divided  into  two  sub- 
orders : 


INSECTA 


103 


Suborder  1.  Rhopalocera,  butterflies  and  skippers,  in 
which  there  is  a  knob  or  swelling  at  the  tip  of  each  antenna. 
Skippers  have  also,  in  addition  to  the  knob,  a  terminal 
recurved  point. 


FIG.  51. — Lepidoptera.     At  the  left  a  cecropia  moth  with  its  eggs,  larva,  and 
cocoon;  at  the  right  a  monarch  butterfly  with  eggs,  larva,  and  chrysalis. 

Suborder  2.     Heterocera,  moths,  which  have  no  knobs 
at  the  ends  of  the  antennae. 

The  colors  of  butterflies  and  moths  are  often  very  beauti- 


104  GENERAL  ZOOLOGY 

ful.  They  are  produced  by  the  little  scales  which  cover  the 
wings  and  other  parts  of  the  body.  They  may  be  due  to 
the  presence  of  specific  pigments  which  give  a  particular 
tint  to  the  surface,  or  they  may  be  " refraction  colors" 
caused  by  the  spreading  of  the  light  from  the  many  small 
flat  surfaces  presented  by  the  scales.  Reds,  yellows,  and 
browns  are  usually  pigment  colors,  while  greens  and  blues 
are  more  often  due  to  refraction. 

The  development  of  Jepidopterous  insects  (Fig.  51)  may 
well  illustrate  the  holometabolous  type;  the  monarch 
butterfly  and  cecropia  moth  serving  as  examples.  The 
monarch  lays  its  eggs  in  little  groups  on  the  underside  of 
milkweed  leaves.  These  eggs  produce  minute  larvae  which 
at  once  begin  to  feed  on  the  leaf  where  they  hatch.  The 
larvae  grow  rapidly,  moulting  several  times,  and  when 
mature  measure  an  inch  or  more  in  length.  They  then 
attach  themselves  in  a  suitable  place,  moult  the  larval  skins, 
and  become  naked  pupae,  or  chrysalids,  which  hang  sus- 
pended until  they  in  turn  moult  to  become  imagoes,  or  adult 
butterflies.  The  cecropia  moth  lays  its  eggs  in  the  spring 
on  willows,  and  the  larvae  which  hatch  feed  voraciously  on 
the  leaves.  When  grown  each  of  them  spins  a  cocoon  about 
itself  and  passes  through  the  winter  attached  to  some 
branch,  safe  inside  its  silken  covering.  In  the  spring  an 
adult  moth  emerges  from  the  pupal  skin  and  forces  its  way 
out  of  the  cocoon.  The  imago  of  this  moth  never  eats  and 
its  mouth  parts  are  rudimentary.  Its  chief  work  in  life  is 
to  find  a  mate  and  start  a  new  generation. 

There  are  many  butterflies  of  particular  interest. 
Among  the  most  beautiful  are  the  swallowtails  (Family 
Papilionidce)  which  have  slender  appendages  from  the 
posterior  portion  of  the  wings.  The  mourning-cloak 
butterfly  (Euvanessa  antiopa)  is  one  of  the  harbingers  of 
spring  in  most  parts  of  the  United  States.  Its  larva  often 
does  damage  to  poplar  and  willow  trees.  The  "  monarch" 
(Anosia  plexippus)  and  ''viceroy"  (Basilarchia  archippus) 
furnish  an  excellent  case  of  mimicry.  The  monarch  is  dis- 


INSECTA  105 

tasteful  to  birds;  its  red  color  gives  notice  of  its  undesira- 
bility,  and  it  generally  is  let  alone.  The  viceroy  lives 
on  the  monarch's  "  reputation. "  It  is  apparently  of  good 
flavor,  for  it  is  readily  eaten  by  birds  if  its  color  is  rubbed 
off.  Its  color  and  general  appearance,  however,  are  very 
much  like  those  of  the  monarch,  and  it  is  consequently 
avoided  by  predaceous  animals.  Probably  the  commonest 
butterfly  in  the  United  States  is  the  cabbage  butterfly, 
Pieris  rapce,  which  is  white  with  black  spots  on  its  wings. 
Its  larva  is  the  green  caterpillar  which  is  often  so  destruc- 
tive to  cabbages. 

Moths  are  of  much  more  economic  importance  than 
butterflies.  Many  species  do  injury  and  some  are  highly 
beneficial.  Among  destructive  larvae  may  be  mentioned: 
the  web  worm,  on  trees  and  shrubs,  tent  caterpillars  (trees), 
cotton  worm,  tobacco  worm,  army  worm  (garden  crops), 
boll  worm  (cotton),  canker  worm  (fruit  trees),  codling 
worm  (apple),  and  the  larvae  of  the  gypsy  (trees),  tussock 
(trees),  clothes,  flour,  and  grain  moths.  The  larvae  of  the 
brown-tailed  moth  has  lately  attracted  considerable  atten- 
tion in  the  Eastern  United  States.  A  person  who  accident- 
ally touches  one  of  the  caterpillars  may  have  poisonous 
hairs  imbedded  in  the  skin,  and  a  festering  eruption  results. 

The  silkworm,  Bombyx  mori,  stands  preeminent  among 
beneficial  insects.  This  larva  is  fed  on  mulberry  leaves  and 
after  attaining  its  growth  spins  the  cocoon  in  which  it 
becomes  a  pupa.  This  cocoon  is  the  only  source  of  com- 
mercial silk.  The  silk  industry  originated  in  China  but 
the  propagation  of  silk  worms  is  now  carried  on  in  most 
parts  of  the  world  where  conditions  are  suitable.  To  ob- 
tain silk,  the  cocoons  are  plunged  into  hot  water  and  un- 
wound, thus  unraveling  the  fine  threads  which  originated 
in  the  spinning  glands  of  the  silk  moth  larva.  Silk  indus- 
tries in  the  United  States  amount  to  about  $100,000,000  a 
year. 


CHAPTER   X 

PHYLUM  ARTHROPODA,  CLASS  INSECTA 

(Continued) 

Order  16.  Diptera,  Flies. — This  great  order  includes  the 
insects  with  two  wings  (the  flies,  mosquitoes,  gnats,  etc.) 
and  a  few  degenerate  species  which  have  no  wings.  The 
hind  wings  are  usually  represented  by  a  pair  of  halteres,  or 
poisers,  the  mouth  parts  are  fitted  for  sucking  or  piercing, 
and  the  metamorphosis  is  complete.  More  than  forty 
thousand  species  of  Diptera  have  been  described  and  prob- 
ably 300,000  more  are  as  yet  unknown.  Howard  says: 
"Not  only  have  the  true  flies  a  superiority  in  point  of 
numbers,  but  entomologists  are  concluding  that  they  prob- 
ably stand  at  the  head  of  the  insect  system  in  point  of 
evolution,  that  is  to  say,  they  are  the  most  specialized 
of  insects. "  They  are  also  of  great  importance  as  disease 
carriers.  There  are  two  suborders: 

Suborder  1.     Diptera  genuina;  true  flies. 

Section  1.     Nematocera  (Fig.  52);  long-horned  flies:  mosquitoes, 

crane  flies,  gall-gnats,  midges,  black-flies. 
Section  2.     Brachycera  (Fig.  53);  short-horned   flies:   horse-flies, 

bee-flies,  robber-flies,  house-flies,  bot-flies,  flower-flies. 
Suborder  2.     Pupipara;  "ticks"  and  sucking  lice.     All  parasitic. 

Crane  flies  (Tipulidce)  look  like  big  mosquitoes.  They 
are  commonly  seen  in  late  summer  in  pastures  and  woods. 
The  larvae  of  most  species  live  in  the  earth  but  some  exist 
in  water,  in  rotten  wood,  or  even  upon  the  leaves  of  plants. 

The  mosquitoes  (Culicidce)  are  of  particular  interest  to 
man.  They  not  only  cause  inconvenience  and  irritation 
by  their  bites,  but  also  carry  some  of  the  most  important 
of  human  diseases.  The  eggs  of  most  mosquitoes  are  laid 
on  water  (Fig.  52).  They  hatch  into  little  "wigglers," 

106 


INSECTA 


107 


which  after  about  a  week  are  in  turn  transformed  into  awk- 
ward pupae.  Though  the  larva  and  pupa  live  in  water, 
they  breathe  air  and  must  therefore  come  to  the  surface 
from  time  to  time.  The  adult  mosquito  lives  a  longer  or 
shorter  time,  depending  on  the  amount  of  food  it  is  able  to 
secure  and  how  soon  it  finds  a  mate.  "Some  species  are  able, 
under  favorable  conditions,  to  pass  through  their  entire  life 
cycle  in  a  week  (egg,  one  day;  larva,  four;  pupa,  two;  adult, 


FIG.  52. — Diptera,  Nematocera.     Crane    fly    and    larva;  mosquito  with 
larva,  and  pupa;  willow-cone  gall  fly;  midges  with  larva  and  pupa. 


variable).  Mosquitoes  are  very  prolific  breeders;  it  is 
possible  for  one  pair  to  produce  two  hundred  million  in 
four  months.  An  adult  mosquito  may  live  for  five  months, 
perhaps  longer.  It  is  thus  able  to  exist  long  after  breed- 
ing ponds  have  dried  up,  and  can  hibernate  during  the 
winter.  Male  mosquitoes  do  not  suck  blood.  They  usu- 
ally subsist  on  plant  juices,  and  will  sip  water,  wine,  etc. 


108  GENERAL  ZOOLOGY 

A  female  may  also  eat  plant  juices,  but  requires  a  meal  of 
blood  before  she  can  ripen  her  eggs. 

Among  important  mosquito-borne  diseases  are  malaria, 
yellow  fever,  filaria,  and  dengue  fever.  Malaria  is  caused 
by  parasitic  animals  (Plasmodium)  which  live  in  the  red 
blood  corpuscles  of  man  and  in  mosquitoes.  It  will  be 
considered  in  detail  in  Chapter  XII;  it  suffices  to  say  here 
that  it  is  carried  only  by  mosquitoes  of  the  family  Ano- 
phelince.  Man  can  get  malarial  fever  only  through  the  bite 
of  such  a  mosquito.  The  genus  Stegomyia  is  the  sole  car- 
rier of  yellow  fever.  Filaria  is  a  parasite  worm  (page  187), 
causing  the  horrible  disease  known  as  elephantiasis,  in 
which  parts  of  the  body  become  enormously  enlarged. 
This  malady  is  also  inoculated  hypodermically  into  the 
bodies  of  unsuspecting  persons  by  certain  mosquitoes  while 
sucking  the  blood.  The  commonest  mosquito  in  the  United 
States,  Culex,  carries  bird  malaria  but  does  not  transmit 
any  human  diseases. 

The  best  way  to  treat  mosquito-borne  diseases  is  by 
preventive  measures — i.e.,  better  not  to  have  the  disease 
than  to  try  to  get  \yell  after  it  has  been  acquired.  This 
means  fighting  mosquitoes,  and  in  warm  countries  where 
such  diseases  are  prevalent,  every  intelligently  managed 
economic  project  has  a  staff  of  men  who  devote  their  whole 
time  to  general  sanitation.  The  building  of  the  Panama 
Canal  depended  as  much  upon  the  efficient  work  of  doctors 
and  zoologists  as  it  did  upon  the  efforts  of  engineers,  steam 
shovels,  and  laborers.  Mosquito  larvse  are  eliminated  from 
particular  districts  by  putting  oil  on  the  water;  by  intro- 
ducing natural  enemies  of  the  larvse,  such  as  sticklebacks, 
top-minnows,,  and  other  fish;  and  by  the  elimination  of 
breeding  places  (filling  in  or  draining  swamps,  covering 
reservoirs,  etc.).  The  adults  are  kept  from  houses  by 
fumigation,  screening,  locally  applied  "  dopes,"  etc. 

The  midges  (Chironomidce)  are  of  great  economic  impor- 
tance, chiefly  because  they  occur  in  such  numbers  as  to 
furnish  an  easily  obtained  food  for  many  animals.  The 


INSECTA  109 

larvae  are  aquatic  and  some  are  known  as  " blood- worms" 
on  account  of  their  color.  Fishes  commonly  feed  on  them 
and  on  midge  pupae.  Many  birds,  insects,  and  spiders 
eat  the  adult  midges,  which  often  occur  in  great  swarms, 
covering  trees,  buildings,  and  everything  else  over  large 
areas.  Such  swarms  are  more  apt  to  come  in  the  spring 
than  at  other  seasons  of  the  year.  Though  some  of  the 
midges  resemble  mosquitoes  in  general  appearance,  most 
species  are  perfectly  harmless.  The  larvae  live  mostly  on 
aquatic  vegetation  and  the  adults  usually  do  not  suck 
blood.  The  minute  "punkie"  or  "no-see-um,"  of  the 
northern  woods  is  an  exception,  as  those  who  have  been 
punctured  by  these  minute  pests  can  testify. 

The  gall-gnats  (Cecidomyiidce)  resemble  mosquitoes  in 
appearance  but  do  not  bite.  Their  larvae  differ  consider- 
ably in  their  habits,  but  those  of  most  species  form  galls 
on  the  twigs  or  leaves  of  plants.  The  most  notable  species 
of  the  family  is  the  Hessian  fly,  Cecidomyia  destructor; 
which  does  many  millions  of  dollars  worth  of  damage  annu- 
ally to  the  wheat  crop  of  the  United  States.  Another  com- 
mon species  forms  curious  galls  on  willows,  which  resemble 
pine  cones  in  shape  (Fig.  52).  Others  live  on  the  golden 
rod,  sunflower,  aster,  and  other  plants.  The  little  black- 
flies,  sand  flies,  or  buffalo-gnats  (Simuliidce)  are  stout, 
hump-backed  Diptera  which  are  great  pests  in  many  parts 
of  the  world.  Their  larvae  live  in  swift  flowing  streams. 
The  adults  are  extremely  blood-thirsty;  cases  are  known 
in  which  cattle,  and  once  even  a  man,  were  killed  by  the 
bites  of  these  little  insects. 

Among  the  short-horned  flies  (Brachycera,  Fig.  53) 
the  horse-flies  (Tabanidce)  may  first  be  considered.  These 
insects  are  also  known  as  gad-flies,  deer-flies,  breeze-flies, 
etc.  They  are  active,  strong-flying  creatures.  The  females 
live  by  sucking  blood  from  warm-blooded  animals,  but  the 
males  are  harmless  and  get  their  sustenance  from  plant 
juices.  Tabanids  usually  lay  their  eggs  above  water  on 
cat-tails,  rushes,  and  other  aquatic  plants.  The  larvae- are 


110 


GENERAL  ZOOLOGY 


carnivorous  and  live  either  in  or  near  the  water.  The  bee- 
flies  (Bombyliidce)  superficially  resemble  bees,  having 
spotted  or  banded  wings  and  hairy  bodies.  They  frequent 
flowers  and  their  long  proboscis  enables  them  to  get  nectar 
and  pollen.  Their  larvae  are  often  parasitic  on  noxious 
insects,  such  as  caterpillars  and  grasshoppers,  and  hence  are 
beneficial  to  man.  The  robber-flies  (Asilidae,  Fig.  53) 
are  veritable  hawks  among  insects.  They  are  slender- 
bodied  creatures  which  fly  with  great  agility,  often  captur- 
ing insects  larger  than  themselves  on  the  wing.  Though 


FIG.  53. — Diptera,  Bracyccra.     Flower-fly,  drone-fly   and  its  "rat-tail"  larvae, 
robber-fly,  flesh-flics  and  maggots  on  fish,  horse  fly  with  larva  and  eggs. 

generally  beneficial,  they  sometimes  do  injury  to  bee- 
keepers and  are  known  in  some  parts  of  the  country  as 
"  bee-killers."  The  bot-flies  (GEstridce)  are  parasitic  and 
have  unique  habits.  The  larvae  live  in  the  bodies  of  living 
animals;  the  common  one  in  the  horse  being  found  in  the 
stomach;  those  of  man,  the  cow,  and  rabbit  beneath  the 
skin.  The  tachina  flies  (Tachinidce)  do  much  good  by 
destroying  caterpillars  and  other  insects.  They  lay  their 


INSECTA 


111 


eggs  in  the  bodies  of  their  hosts  and  the  larvae  live  as  in- 
ternal parasites.  The  flesh-flies  or  blow-flies  (Sarcopha- 
gidce)  are  commonly  found  about  decaying  meat.  The 
maggots  burrowing  through  their  food;  the  adults  eat  and 
lay  eggs  on  the  outside.  The  larvae  are  sometimes  found 
in  living  animals,  including  man.  The  flower-flies  (Syr- 


SOURCE 
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Fia.  54. — Showing  the  relations  of  flies  to  disease  and  a  cartoon  from  a  current 

newspaper. 

phidce)  frequently  are  abundant  in  the  United  States. 
They  fly  about  on  bright  days  and  rest  at  other  times  in 
flowers  or  among  leaves.  Their  food  varies  greatly  in  the 
larval  state,  but  as  adults  most  species  subsist  on  pollen 
and  nectar. 

The  house-flies  (Muscidce,  Fig.  54)  are  probably  of  more 


112  GENERAL  ZOOLOGY 

importance  to  man  than  any  other  group  of  insects.  The 
commonest  species  are  the  house-fly,  Musca  domestica,  and 
the  stable-fly,  Stomoxys  calcitrans.  The  horn-fly  of  cattle, 
Hcematobia  serrata,  is  also  well  known  to  farmers.  Many 
flies  belonging  to  this  family  are  of  value  as  scavengers, 
destroying  decaying  matter  through  their  great  numbers 
and  rapid  breeding.  The  horn-fly  and  stable-fly  produce 
great  losses  in  the  dairy  and  stock  industries  by  their 
attacks  on  cattle.  The  common  house-fly,  and  its  near 
relatives,  do  untold  injury  to  man.  The  facility  with 
which  these  insects  take  to  civilized  life  and  their  uncleanly 
habits  make  them  ideal  carriers  of  disease  germs.  The 
house-fly  commonly  breeds  in  the  excreta  of  diseased  men  or 
other  animals,  and  its  roving  habits  make  it  a  continual 
source  of  contamination.  Many  bacteria  will  pass  through 
its  alimentary  canal  without  losing  their  virulence.  Thus 
a  house-fly  not  only  goes  about  with  dirty  body  fouling 
everything  it  touches,  but.  is  continually  defecating  and 
spitting  out  germ  cultures  as  it  goes.  Typhoid  fever, 
diarrhoea,  and  cholera  are  commonly  carried  by  house- 
flies.  Tuberculosis,  infantile  paralysis,  anthrax,  and  yaws 
may  spread  in  the  same  way.  Perhaps  syphilis,  eye  dis- 
eases, leprosy,  diphtheria,  small  pox,  plague,  and  tropical 
sores  may  also  be  carried  by  flies  in  some  cases  (Fig.  54). 
Small  wonder  that  the  house-fly  is  the  most  dreaded  hanger- 
on  of  civilization  and  that  many  communities  have  taken 
active  measures  to  keep  it  down. 

Order  17.  Siphonaptera,  Fleas  (Fig.  55). — These  in- 
sects show  many  adaptations  to  parasitic  life  on  mammals. 
They  are  greatly  compressed,  so  that  they  are  not  easily 
squeezed  to  death  and  may  easily  ramble  about  among 
hairs.  The  mouth  parts  are  suitable  for  sucking  blood. 
There  are  no  wings,  but  the  powers  of  jumping  are  re- 
markably developed.  A  flea  is  thus  able,  if  it  drops  off 
one  animal,  to  hop  on  another  when  it  comes  near.  Fleas 
lay  their  eggs  in  cracks  in  floors  and  similar  situations. 
The  larvae  eat  sweepings,  or  almost  any  sort  of  waste 


INSECTA 


113 


material,  such  as  may  accumulate  about  the  abodes  of 
animals.  Adult  fleas  feed  on  various  warm-blooded  ani- 
mals. Dogs,  cats,  rats,  men  and  other  animals  have 
characteristic  species  associated  with  them.  Most  spe- 
cies of  fleas  prefer  a  particular  host/ but  will  feed  on  others 
if  hungry. 

Fleas  have  lately  excited  interest  among  medical  men 
because  they  have  been  found  to  be  carriers  of  several 
diseases.  The  rat  flea,  Xenophylla  cheops,  carries  the 
bubonic  plague.  This  dreadful  disease  has  several  times 
become  established  on  the  borders  of  the  United  States, 
being  introduced  on  vessels  from  foreign  countries.  Pre- 
ventative  measures  consist  in  the  destruction  of  rats  and 


Fio.  55. — A  flea  with  its  eggs,  larva,  and  cocoon. 

other  rodents,  placing  metal  "rat  guards"  on  the  cables  of 
ships  while  in  port,  fumigation,  etc.  Where  the  plague  has 
become  established,  regular  campaigns  must  be  waged. 
In  California  the  government  has  recently  hired  squads 
of  men  to  destroy  rats  and  ground  squirrels.  In  New 
Orleans  the  houses  in  certain  parts  of  the  city  were  com- 
pletely destroyed  and  replaced  by  " rat-proof"  buildings. 
Order  18.  Coleoptera,  Beetles  (Fig.  56).— This  order 
approaches  the  Diptera  and  Lepidoptera  in  numbers. 
There  are  about  twelve  thousand  species  in  the  United 
States.  Beetles  have  four  pairs  of  wings,  the  first  pair 
forming  hard  sheaths,  or  elytra,  which  cover  the  mem- 


114 


GENERAL  ZOOLOGY 


branous  hind  wings.  Their  mouth  parts  are  fitted  for 
biting,  and  metamorphosis  is  complete.  There  are  eight 
suborders : 

Suborder  1.  Adephaga. — The  beetles  in  this  group  are 
all  carnivorous.  Two  important  families  live  on  land  and 
the  other  two  are  aquatic.  The  tiger-beetles  (Cincindelidce) 
are  bright-colored  active  creatures  which  seek  their  prey 


"'- 

&£  -^  — - 


FIG.  56. — Coleoptera.  June-beetles,  their  grub  and  pupa;  potato-beetle; 
lady-bird;  rove -beetle;  tumble  "bug";  plant  slugs;  click-beetle;  predaceous 
ground-beetles;  snout-beetles;  weevils. 

in  open  places,  such  as  roadways  and  sandy  beaches. 
The  predaceous  ground-beetles  (Carabidce)  are  usually 
black  or  brown.  They  search  the  surface  of  the  soil  for 
caterpillars  and  other  small  insects  which  they  attack  with 
great  vigor.  The  predaceous  diving-beetles  (Dytiscidce) 
are  found  among  aquatic  vegetation  where  they  catch 
various  small  animals.  The  whirligig-beetles  (Gyrinidce) 
make  curious  gyrating  movements  on  the  surface  of  the 


INSECTA  115 

ponds  or  rivers.  Their  food  consists  mostly  of  small 
insects  which  fall  into  the  water. 

Suborder  2.  Clavicornia. — The  club-horned  beetles  have 
the  antennae  swollen  at  the  distal  ends.  Their  habits 
vary  greatly.  The  water-scavengers  (Hydrophilidce),  rove- 
beetles  (Staphylindce) ,  and  burying-beetles  (Silphidce)  do 
great  good  by  disposing  of  dead  animals.  The  grain 
beetles  (Cucujidce)  are  injurious  in  granaries.  The  larder- 
beetles  (Dermestidce)  are  great  pests  in  museums,  factories, 
etc.  They  eat  all  sorts  of  organic  substances,  but  are 
particularly  fond  of  dry  animal  remains,  such  as  insects  and 
furs. 

Suborder  3.  Serricornia. — The  saw-horned  beetles  have 
serrated  antennae.  Here  belong  the  metallic  wood  borers 
(Bubestridce)  which  injure  trees  and  logs;  the  click-beetles 
(Elateridce),  so-called  because  they  can  flip  themselves  over 
when  turned  on  their  backs;  the  fireflies  and  soldier-beetles 
(Lampyridce) ;  and  others. 

Suborder  4.  Lamellicornia. — The  blade-horned  beetles 
have  the  distal  ends  of  their  antennae  flattened.  The  stag- 
beetles  (Lucanidce)  are  probably  beneficial,  as  they  de- 
stroy other  insects.  Their  name  refers  to  the  peculiar 
antler-like  growths  on  the  heads  of  the  males  of  some 
species.  The  family  Scarabceidce  contains  a  number  of 
beetles  which  fall  into  two  groups  as  regards  habits.  The 
scavenger-beetles  eat  decaying  matter.  The  tumble-bug 
is  an  example.  It  places  its  eggs  in  a  ball  of  manure,  which 
it  rolls  to  some  hole  and  buries.  The  Sacred  Scarabaeus 
of  Egypt  belongs  in  this  family.  The  leaf-chafers  are 
injurious  vegetarians;  the  common  June-bug,  Lachnos- 
terna  fusca,  being  an  example.  This  beetle  is  a  biennial. 
It  spends  over  a  year  in  the  ground  as  a  white  grub,  and 
hence  appears  only  every  third  year. 

Suborder  5.  Phytophaga. — The  plant-eating  beetles  in- 
clude the  long-horned  wood-borers  (Cerambycidce)  and  the 
bean  weevils  (Bruchidce).  The  larvae  of  these  insects  are 
able  to  bore  their  way  through  very  resistant  materials. 


116  GENERAL  ZOOLOGY 

The  most  important  family  is  the  Chrysomelidce  which 
contains  most  of  the  common  beetle  pests — the  potato- 
bug,  elm-leaf  beetle,  cucumber-beetle,  etc. 

Suborder  6.  Trimera. — This  order  contains  only  one 
important  family,  the  CoccinellidoBy  or  lady-bird  beetles. 
These  little  insects  both  as  larvae  and  adults,  do  great 
good  to  man  by  feeding  on  aphids  and  scale-insects. 

Suborder  7.  Heteromera. — The  darkling  and  blister- 
beetles  are  of  considerable  economic  importance.  The 
former  (Tenebrionidce)  include  Tenebrio  molitor,  the  larva 
of  which  is  the  " meal- worm"  so  common  about  flour  and 
grain  bins.  One  of  the  blister-beetles  (Meloidce)  when 
dried  and  pulverized,  has  the  peculiar  property  of  raising 
blisters  on  the  human  skin,  and  is  used  by  physicians  for 
that  purpose. 

Suborder  8.  Rhynchophora. — The  snout-beetles  differ 
from  all  others  in  having  the  head  prolonged  into  a  beak, 
with  tiny  mouth  parts  at  its  tip.  The  bark-beetles  (Sco- 
lytidce)  are  most  injurious  of  all  insects  to  forest  trees, 
causing  $100,000,000  worth  of  damage  annually  in  the 
United  States.  The  larvae  make  tunnels  beneath  the  bark 
of  living  or  dead  trees  and  thus  open  the  way  for  rot-  and 
decay-fungi.  The  curculios  and  weevils  damage  fruits 
and  grains. 

Order  19.  Hymenoptera  (Fig.  57). — To  this  order  belong 
the  gall-flies,  ichneumon-flies,  horn-tails,  saw-flies,  wasps, 
bees,  and  ants.  These  insects  possess  four  membranous 
wings  with  few  veins;  their  mouth  parts  are  fitted  for  both 
biting  and  sucking;  the  female  often  has  a  long  ovipositor 
so  modified  as  to  form  a  sting;  the  metamorphosis  is 
complete.  Many  of  these  are  of  unusual  importance  to 
man.  There  are  about  7500  species  in  North  America. 
Only  the  most  important  families  can  be  mentioned. 

The  saw-flies  (Tenthredinidce)  are  so  named  on  account 
of  the  saw-like  ovipositor  possessed  by  the  females.  The 
adults  are  rather  inconspicuous,  but  many  of  the  larvae 
are  well  known  as  rose-,  currant-,  pear-,  larch-,  and  willow- 


INSECTA 


117 


slugs.  The  horn-tails  (Siricidce)  are  related  to  the  saw- 
flies.  The  females  have  a  long  hard  ovipositor  at  the  end 
of  the  abdomen  with  which  they  drill  holes  in  trees.  The 
larvae  burrow  in  the  heart  wood  and  grow  into  white  grubs. 
The  gall-flies  (Cynipidce)  lay  eggs  in  living  buds,  leaves, 
or  stems,  and  their  larvae  develop  inside  galls  which  develop 
on  the  plants  attacked.  Though  such  abnormal  growths 
are  found  on  many  plants  they  are  most  common  on  the 


FIG.  57. — Hymenoptera.     Chalcid-fly    and    cocoons   on    caterpillar;    bees   and 
wasps;  ichneumon-flies;  horn-tail  larva. 

oak,  rose,  and  hickory.  All  the  gall-flies  are  of  small  size, 
the  largest  being  only  a  third  of  an  inch  long.  The  galls 
produced  by  different  species  are  quite  characteristic. 
They  present  a  great  variety  of  forms — rosettes,  stars, 
spheres,  and  other  shapes.  The  growth  of  a  gall  is  due  to 
the  irritation  from  the  developing  larva  within.  Some 
gall-flies  do  not  form  galls  of  their  own,  but  lay  their  eggs 
in  the  growths  produced  by  other  species. 


118  GENERAL  ZOOLOGY 

The  chalcid-flies  (Chalcididce)  do  great  good  to  man  by 
laying  their  eggs  in  insects,  particularly  caterpillars. 
The  eggs  hatch  in  or  on  the  host  and  the  larvae  burrow  about 
among  the  internal  organs.  Sometimes  in  the  summer  one 
may  find  a  dead  caterpillar  covered  with  little  white 
cocoons.  These  contain  the  pupae  of  chalcids  which 
lived  within  the  caterpillar  as  larvae. 

The  ichneumon-flies  (Ichneumonidce)  are  also  parasitic 
on  caterpillars  and  other  insects.  There  are  many  spe- 
cies and  most  of  them  attack  only  one  host.  Thalessa 
lunator  (Fig.  57)  is  an  interesting  parasite  on  the  larvae 
of  the  pigeon  horn-tail,  Tremex  columba.  These  larvae 
are  found  in  the  heart  wood  of  trees.  The  female  Thalessa 
bores  into  a  tree  with  her  slender  ovipositor  and  lays  her 
eggs  in  them. 

There  are  several  families  of  wasps.  The  most  primi- 
tive species  are  diggers,  carpenters,  or  masons.  Some 
tunnel  in  the  ground,  others  make  galleries  in  wood,  or 
plaster  pellets  of  mud  together  to  form  nests.  Such 
cavities  are  stored  with  spiders,  caterpillars,  and  other 
insects.  An  egg  is  deposited  with  accumulated  food  and 
each  cavity  is  sealed  up.  The  sting  of  a  wasp  contains  a 
poison  which  paralyzes  the  nervous  system  of  its  prey. 
The  insects  placed  in  the  cells  with  the  eggs  are  not  killed, 
but  only  paralyzed.  They  thus  remain  in  "cold  storage" 
until  the  wasp's  larva  is  ready  to  devour  them.  The  social 
wasps  (Vespidce)  live  in  colonies.  They  do  not  seal  up  the 
egg  with  stored  food,  but  care  for  and  feed  the  larva  soon 
after  it  hatches.  The  greater  part  of  each  colony  dies 
when  winter  comes.  Single  fertile  females  hibernate  in 
rotten  stumps,  in  barns,  under  logs,  and  in  similar  situa- 
tions. In  spring  each  builds  a  little  nest  and  begins  to  lay 
eggs.  As  soon  as  the  young  wasps  emerge  from  the  pupal 
condition  they  begin  to  help  enlarge  the  nest,  gather  food, 
and  care  for  the  young.  The  original  queen,  the  males, 
and  the  "workers"  (which  are  sexually  immature  females) 
all  die  in  the  autumn.  Before  the  colony  breaks  up,  how- 


INSECTA  119 

ever,  the  males  fertilize  young  females  so  as  to  enable  them 
to  start  colonies  for  the  next  year.  The  two  common 
' '  paper- wasps  "  in  the  United  States  are  Vespa  and  Polistes. 
Their  nests  are  made  of  chewed  vegetable  fibers. 

The  bees  are  included  in  one  great  family,  Apidce.  As 
among  the  wasps  there  are  solitary  and  social  species. 
There  are  also  burro  wers,  carpenters,  leaf-cutters,  etc. 
Bees  may  have  short  or  long  tongues  and  the  food  varies 
accordingly.  The  bumble  bee  is  like  the  social  wasps,  in 
that  most  of  the  colony  dies  with  the  advent  of  winter. 
The  honey-bee,  however,  has  a  perennial  community  which 
gives  off  new  colonies  by  "swarming."  When  swarm- 
ing takes  place  a  queen  rushes  from  her  parent  hive,  fol- 
lowed by  most  of  the  workers  and  the  drones,  or  males. 
She  is  fertilized  but  once;  while  making  this  flight,  and  is 
then  able  to  lay  thousands  of  fertile  eggs.  Scouts  go  out 
from  the  swarm  and  locate  a  good  place  for  a  new  colony. 
The  queen  never  leaves  the  new  home,  but  spends  her  time 
going  about  laying  eggs  in  the  comb  which  the  workers 
construct.  She  is  fed  and  cared  for  by  the  workers,  but 
usually  dies  after  three  or  four  years  and  is  replaced  by  a 
young  queen.  The  bee-industries  in  the  United  States 
are  of  considerable  importance.  About  seventeen  million 
dollars  worth  of  honey  and  wax  are  produced  annually. 

The  ants  (Formieidce,  Fig.  58)  are  remarkable  for  their 
development  of  social  instincts.  They  have  gone  far 
ahead  of  the  bees  in  this  respect,  and  even  excel  the  ter- 
mites. The  queens  start  the  colonies,  as  among  other 
social  Hymenoptera,  and  the  workers  are  all  sterile  females. 
Polymorphism  reaches  great  extremes  in  all  the  castes. 
The  queens  and  males  may  be  large  or  small,  and  with  or 
without  wings.  The  worker  caste  may  have  several 
varieties,  such  as  large  and  small  workers,  soldiers,  door- 
keepers, crop  harvesters,  and  nurses. 

Ants  carry  on  many  complicated  activities  which  involve 
the  cooperation  of  all  members  of  the  colony.  The  leaf- 
cutters  of  tropical  America  build  large  nests  in  forests. 


120  GENERAL  ZOOLOGY 

From  these  the  workers,  under  protection  of  soldiers,  go 
forth  in  great  armies  along  beaten  paths.  They  seek  out 
particular  plants  from  which  the  leaves  and  stems  are 
methodically  snipped  off.  The  booty  is  carried  back  to 
the  nest,  taken  underground,  and  chewed  into  fine  bits 
by  another  caste.  The  soft  finely  divided  mass  is  then 
spread  out  in  underground  beds  and  planted  with  a  pecu- 
liar fungus  which  is  the  sole  food  of  leaf  cutters.  The 
particular  species  of  fungus  used  by  the  ants  has  never 
been  found  except  in  their  nests.  It  is,  therefore,  as  much 


Fio.  68. — An  ant's  nest  showing  queen,  workers,  soldiers,  eggs,  larvae,  pupae, 

and  beetle  guests. 

a  "tame"  or  cultivated  crop  as  the  cabbages  and  potatoes 
of  men.  When  a  young  queen  is  ready  to  start  a  new 
colony  she  takes  a  little  pellet  from  the  fungus  bed  and 
places  it  in  a  little  pocket  at  the  back  of  her  mouth.  She 
then  goes  to  some  favorable  locality,  digs  a  little  burrow  in 
the  ground,  cuts  and  chews  a  few  leaves,  and  makes  a  little 
bed.  The  pellet  is  taken  from  her  mouth  and  planted  on 
the  bed.  All  these  operations  take  a  month  or  more  and 
during  this  time  the  queen  takes  no  food.  She  now  lays 
eggs  and  cares  for  the  larvae  which  hatch  from  them.  As 


INSECTA  121 

soon  as  the  youngsters  mature,  however,  she  does  no  more 
work,  but  is  fed  and  kept  clean  by  them. 

The  large  red  mound-building  ant,  Formica  rufa,  found 
in  various  parts  of  the  United  States,  commonly  enslaves 
other  species.  A  suitable  nest  is  located  in  advance  and 
the  whole  colony  raids  it — killing  the  soldiers  and  carrying 
away  the  workers,  larvse  and  pupae.  Some  tropical  ants 
are  so  dependent  upon  slaves  that  they  are  not  even  able 
to  feed  themselves.  The  harvesting  ants  collect  certain 
seeds  which  they  husk,  dry,  and  store  in  little  underground 
granaries. 

The  complex  communities  of  ants  all  center  about  the 
queen.  She  is  always  guarded,  cleaned,  and  fed  with  great 
care.  She  must  be  kept  in  good  condition,  for  the  con- 
tinued existence  of  the  colony  depends  upon  her  ability  to 
lay  eggs.  The  males  are  of  no  service  to  the  community 
except  to  fertilize  the  queen.  The  workers  carry  on  all 
general  activities.  They  gather  food  and  feed  each  other, 
fight,  dig,  gnaw,  clean,  nurse,  etc.  The  instinct  to  re- 
gurgitate food  from  the  crop  to  feed  other  ants  is  strongly 
developed  and  is  probably  largely  responsible  for  the 
extreme  development  of  castes  among  ants.  It  makes  it 
possible  for  one  caste  to  do  all  the  food  collecting  and  feed- 
ing, while  other  individuals  become  specialized  for  other 
occupations. 

Yet,  notwithstanding  the  complex  activities  of  ants, 
they  have  little  "  intelligence."  They  follow  their  in- 
stincts slavishly  and  are  not  able  to  solve  the  simplest 
problems.  Different  individuals  of  a  colony  recognize  each 
other  largely  by  odor.  Experiments  have  been  performed 
in  which  strange  ants  were  smeared  over  with  the  extract 
from  the  bodies  of  certain  individuals  from  a  particular 
colony.  When  these  disguised  individuals  were  put  in  the 
nest  from  which  their  odor  came,  they  \vere  received  as 
friends.  In  another  experiment  an  ant  was  taken  from 
a  nest  and  its  odor  was  washed  off.  On  being  returned  to 
its  own  nest,  it  was  seized  and  torn  to  pieces. 


122  GENERAL  ZOOLOGY 

This  blind  following  of  reflexes  and  instincts  makes  ants 
much  subject  to  deception  by  other  animals.  Such  ras- 
cals live  as  social  parasites  in  ant  communities  and  are  not 
ejected — though  they  do  no  work — because  they  have  the 
proper  odor  or  some  other  quality  which  takes  advantage 
of  the  ant's  simple  instincts.  More  than  fifteen  hundred 
species  of  ants'  guests,  or  myrmeeophiles,  are  known. 
These  include  beetles,  isopods,  and  other  arthropods. 
Many  of  them  are  not  known  to  occur  except  in  ant's 
nests  (Fig.  58). 


CHAPTER  XI 

PHYLUM  ARTHROPODA,  CLASS  ARACHNIDA 

The  arachnids  differ  from  all  other  arthropods  in  having 
the  following  combination  of  characters:  (1)  no  antennae; 
(2)  no  true  jaws,  but  only  a  pair  of  claw-like  appendages 
on  either  side  of  the  mouth;  (3)  the  head  and  thorax  are 
combined  to  form  a  cephalothorax,  which  bears  the  four 
walking  legs  and  is  usually  distinct  from  the  abdomen. 
There  are  twelve  orders  in  the  Class  Arachnida  but  only 
four  will  be  considered.  The  other  eight  contain  animals 
which  are  rare,  minute  in  size,  or  confined  to  tropical 
countries. 

ORDER  1.    ARANEIDA,  SPIDERS 

The  true  spiders  (Fig.  59)  have  two  body  regions — the 
cephalothorax  and  abdomen.  The  cephalothorax  is  rec- 
tangular and  flattened;  the  abdomen  is  unsegmented,  soft, 
and  rounded.  The  spiders  are,  therefore,  readily  distin- 
guished from  the  other  common  arachnids.  The  ticks  and 
mites  have  no  division  of  the  body  into  regions;  in  the 
scorpions  the  body  is  flattened  and  the  abdomen  is  very 
slender;  and  in  the  phalangids,  or  "daddy-long-legs,"  the 
appendages  are  slender  and  the  body  small. 

The  class  Arachnida  received  its  name  on  account  of  the 
spinning  ability  of  the  spiders.  It  is  related  in  ancient 
mythology  that  Arachne  was  a  mighty  spinner;  so  good,  in 
fact,  that  she  became  conceited  and  boasted  that  she  could 
beat  Minerva  herself.  That  great  goddess,  after  spinning 
and  winning  a  match  with  the  foolish  Arachne,  turned  her 
into  a  spider.  And  thus,  says  the  legend,  spiders  had  their 
beginning.  Modern  zoology  has  kept  Arachne's  memory 
green  by  naming  spiders  and  related  arthropods  for  her, 

123 


124 


GENERAL  ZOOLOGY 


but  only  the  true  spiders  are  able  to  spin.  In  order  to  give 
some  idea  of  the  daily  life  of  a  spider,  the  activities  of  the 
common  garden  spider  will  now  be  considered. 


THE  GARDEN  SPIDER,  Miranda  aurantia  Lucas 

Self -maintenance. — The  golden  garden  spider  (Fig.  59) 
spreads  its  beautiful  orbs  on  low  shrubs,  bushes,  or  even 
among  tall  grasses.  These  symmetrical  webs  are  traps 


FIG.  59. — Spiders.  Orb  weavers  and  cocoon;  ground  spider  carrying  its  co- 
coon; jumping  spider;  young  individual  ready  to  "balloon";  a  tent  weaver;  a 
wasp  flying  to  its  nest  with  a  spider;  crab-spider  lurking  in  a  flower. 

set  to  catch  prey.  Their  building  and  maintenance  re- 
quire a  great  deal  of  work.  .  They  are  made  from  "  spider- 
web,"  or  silk,  which  is  elaborated  by  little  glands  within 
the  spider's  abdomen  and  discharged  through  the  six 
finger-like  spinnerets  at  the  posterior  end.  The  web  is  a 
viscid  fluid  within  the  body,  but  hardens  into  threads  on 


ARACHNIDA  125 

exposure  to  the  atmosphere.  Spider's  silk  is  very  strong, 
and  yet  flexible.  It  is  used  by  man  for  several  purposes — 
as  cross  lines  in  surveyor's  instruments,  to  swing  the 
pendulums  of  very  delicate  and  accurate  clocks. 

It  is  very  interesting  to  watch  an  orb- weaver  spin  its 
web.  First  it  climbs  to  some  favorable  spot  and  sends  a 
floating  strand  of  silk  out  on  the  wind.  If  the  other  end 
catches,  the  spider  runs  along  the  line  several  times, 
strengthening  the  first  thread  by  adding  other  strands; 
then  goes  to  the  middle  of  its  line,  attaches  a  new  strand, 
and  drops  to  the  ground,  thus  forming  a  new  line  from  the 
middle  of  the  first  to  the  ground.  By  letting  go  new 
threads,  and  dropping  down,  and  scrambling  up,  a  couple 
of  dozen  lines  are  stretched.  These  meet  at  a  point  which 
is  to  be  the  center  of  the  orb.  After  these  " radii"  are 
ready,  a  continuous  spiral  is  spun  from  the  center  outward 
which  forms  a  series  of  concentric  cross  connections  between 
them.  An  orb-weaver  is  able  to  spin  two  kinds  of  silk, 
one  of  which  is  covered  with  little  viscid  droplets.  Finally, 
two  viscid  threads  are  put  on  the  radii  between  the  con- 
centric curves  of  the  spiral. 

The  snare  is  now  ready,  and  the  spider  takes  position  at 
the  center,  ready  to  dart  out  in  any  direction.  If  an  awk- 
ward grasshopper  or  a  careless  fly  blunders  into  the  web, 
the  viscid  threads  stick  to  it  until  the  spider  can  run  out 
and  quiet  the  struggling  victim  by  swathing  it  in  silk.  A 
spider,  crouching  at  the  center  of  its  web,  is  able  to  see 
well  in  all  directions,  for  there  are  eight  little  eyes  on  the 
top  and  front  of  its  head.  It  is  able  to  observe  and  pursue 
its  prey  with  precision.  The  claws  at  the  tips  of  the  legs 
are  admirably  adjusted  for  running  over  the  web  and 
handling  silk  threads.  Any  animal  captured  is  seized  in 
the  jaws  and  the  secretion  from  the  poison  claws  at  their 
tips  soon  quiets  it. 

A  spider  has  a  small  mouth  and  cannot  eat  large 
pieces  of  food.  It  sucks  the  juices  of  its  victims  and  casts 
their  shriveled  bodies  out  of  its  web.  At  the  end  of  the 


126  GENERAL  ZOOLOGY 

esophagus  there  is  a  muscular  sucking  stomach  which  pumps 
the  nutritive  fluids  from  the  prey.  The  food  is  digested 
to  some  extent  in  the  large  true  stomach,  and  then  passes 
on  into  the  intestine  to  be  acted  upon  further  and  absorbed. 
There  are  five  large  sacs,  or  caeca,  connected  with  the  stom- 
ach. These  serve  to  increase  its  digestive  and  absorp- 
tive surface.  Digestion  is  also  assisted  in  the  intestine  by 
a  number  of  pouches  which  branch  out  to  such  an  extent 
that  they  fill  most  of  the  space  within  the  abdomen. 

After  being  absorbed  the  food  is  taken  into  the  circulatory 
system ;  passes  through  sinuses  and  veins  to  the  pericardial 
chamber.  A  spider's  heart  is  wholly  within  the  abdomen 
and  is  fastened  to  the  walls  of  the  chamber  in  which  it 
lies  by  little  elastic  threads.  These  pull  it  out  into  an  ex- 
panded condition  after  each  contraction.  While  expanding 
the  heart  fills  with  blood  through  three  pairs  of  lateral 
slits,  or  ostia.  During  contraction  the  blood  is  forced  out 
through  the  arteries  to  all  parts  of  the  body.  Backflow 
through  the  ostia  is  prevented  by  valves.  Most  of  the 
blood  coming  to  the  heart  has  been  purified  by  passing 
through  the  "leaves"  of  the  book  lungs,  which  are  the 
respiratory  organs  of  spiders.  There  is  a  pair  of  openings 
on  the  ventral  side  of  the  spider's  abdomen,  which  lead 
into  little  chambers  within  the  body  in  which  the  lung- 
books  lie.  Air  is  drawn  in  through  the  openings  and 
passes  between  the  leaves  of  the  book,  thus  giving  the  blood 
within  a  chance  to  give  up  waste  gases  and  acquire  oxygen 
through  the  thin  epidermal  covering.  The  excretory 
organs  of  spiders  consist  of  slender  Malpighian  tubules. 
Their  free  ends  are  closed  and  lie  among  the  internal 
organs.  Their  proximal  ends  open  into  the  intestine,  and 
there  discharge  absorbed  waste  products. 

Self -protection. — Spiders,  like  most  predaceous  animals, 
are  solitary  in  their  habits.  They  usually  sit  quietly  in 
their  webs  waiting  for  prey.  If  danger  threatens,  a  spider 
quickly  seeks  some  retreat  or  drops  to  the  ground.  Before 
it  goes,  however,  a  thread  is  attached  and  a  silken  strand, 


ARACHNIDA  127 

the  ground  line,  is  always  passed  out  as  the  spider  flees. 
After  danger  is  past,  the  spider  is  able  to  return  quickly 
and  easily  to  its  web  by  taking  in  the  ground  line. 

The  chief  enemies  of  spiders  are  the  mud-dauber  wasps. 
One  of  these  agile  insects  is  able  to  pull  an  orb-weaver 
from  its  web  and  fly  away  without  being  entangled.  The 
luckless  victim  of  the  raid  is  rendered  torpid  by  stinging 
and  used  to  provision  the  little  earthen  cells  in  which  the 
wasp's  eggs  are  laid.  The  spider  guards  against  the  wasp's 
attacks  by  spinning  an  irregular  tangle  of  threads  on  all 
sides  of  its  orb.  These  outlying  lines  impeded  the  prog- 
ress of  winged  aggressors,  or  by  their  vibrations  at  least 
give  warning  that  an  attack  is  impending. 

Spiders  are  subject  to  certain  parasites  and  diseases 
which  sap  their  vitality  or  destroy  them  outright.  Such 
afflictions  are  more  prevalent  during  damp  weather.  For 
'many  reasons  rain  is  one  of  the  spider's  chief  obstacles  to 
success  in  life.  Raindrops  or  heavy  dews  break  down  the 
web  and  on  this  account  continual  repairs  are  necessary. 

Race  Preservation. — Spiders  are  of  two  sexes,  male  or 
female.  The  latter  is  larger  and  spins  the  more  perfect 
webs.  When  mature  the  little  male  frequents  the  webs  of 
females  in  search  of  a  mate  (Fig.  59).  The  males  of  some 
species  go  through  various  fantastic  courting  activities, 
such  as  dancing,  posturing,  and  posing  before  the  female. 
Being  a  chosen  suitor  is  a  somewhat  doubtful  honor,  how- 
ever, for  the  female  often  eats  the  male,  sometimes  even 
before  she  has  been  fertilized. 

After  a  female  has  received  sperm  cells  for  fertilizing 
her  eggs,  she  usually  makes  elaborate  preparations  for 
cocooning.  Some  species  of  spiders  carry  their  cocoons 
about  in  their  jaws,  or  attach  them  to  their  abdomens; 
others  hide  them  away  under  stones  or  logs.  The  orb- 
weaver  goes  to  some  sheltered  nook  in  low  vegetation  and 
fastens  her  cocoon  to  the  plants  (Fig.  59).  The  gauzy 
structure  is  pear-shaped  and  hangs  free  from  surrounding 
objects,  suspended  by  silken  threads.  In  the  center  is  a 


128  GENERAL  ZOOLOGY 

mass  of  eggs  closely  swathed  in  soft  " woolly"  silk  and 
around  this  are  two  or  three  closely  woven  envelopes  for 
protection.  A  mother  spider  usually  dies  soon  after  her 
cocoon  is  finished,  but  the  young  spiders,  hatching  within 
the  envelopes  she  has  furnished,  are  able  to  endure  the 
winter  and  in  the  spring  shift  for  themselves.  The  young 
of  some  spiders  hatch  in  the  spring,  others  in  the  autumn. 
Development  within  the  egg  usually  requires  only  a  few 
days  but  spiderlings  may  remain  inside  the  cocoon  for 
months.  • 

On  a  warm  spring  or  autumn  day  you  will  often  find 
every  tree  and  fence  bearing  long  streamers  of  "  cob  webs." 
These  are  the  " balloons"  of  the  young  spiders.  A  co- 
coon may  produce  fifty  young  and  it  would  not  be  desira- 
ble if  they  should  all  try  to  build  orbs  in  the  same  bush. 
This  difficulty  is  avoided  by  the  ballooning  instincts  of  the 
young.  Soon  after  hatching  they  climb  to  some  elevated 
situation,  each  lets  out  a  long  streamer  of  silk,  and  then 
jumps  off  into  space  (Fig.  59).  The  floating  strands  of 
silk  are  carried  by  the  wind  and  the  spiders  may  travel 
long  distances.  Thus  each  finds  favorable  hunting  grounds, 
remote  from  numerous  brothers  and  sisters.  Spiders  pass 
through  a  series  of  moults  but  there  is  no  metamorphosis. 
The  young  are  much  like  adults  in  their  general  form. 

GENERAL  REMARKS  ON  SPIDERS 

If  spiders  are  compared  with  their  remote  ancestors 
(Figs.  27,  28)  it  is  apparent  that  they  are  highly  specialized. 
The  race  has  acquired  many  improvements  in  structures 
and  methods.  Though  traces  of  segmentation  are  dis- 
cernible in  all  the  body  regions,  there  are  no  free  body 
segments.  To  secure  greater  compactness  and  rigidity 
all  the  segments  have  been  united;  the  head  and  thorax 
have  also  fused  to  form  a  single  region,  the  cephalothorax. 
The  awkward  abdominal  appendages  have  been  completely 
lost,  the  heart  has  become  restricted  to  the  abdomen,  and 
there  are  many  other  evidences  of  racial  specialization. 


ARACHNIDA  129 

It  is  remarkable  with  what  tenacity  different  groups  of 
animals  cling  to  the  " styles"  laid  down  for  them  by  nature. 
There  is  no  particular  reason  why  an  arthropod  should 
have  three  or  four  pairs  of  legs — yet,  there  is  no  adult 
insect  which  does  not  have  three,  and  no  arachnid  without 
four.  Spiders  all  have  spinning  glands  on  the  abdomen; 
insects  possess  such  organs  only  during  larval  stages. 
Many  insects  have  wings;  no  arachnids  bear  such  append- 
ages. The  exact  ancestry  of  spiders  is  somewhat  un- 
certain. The  earliest  spider-like  animals  known  lived  in  the 
shallow  waters  along  the  seashore.  The  fact  that  some 
arachnids  (ticks,  mites)  have  larvae  with  three  pairs  of 
legs,  however,  has  led  some  men  to  believe  that  spiders 
descended  from  insect  stock. 

In  making  adjustments  to  their  surroundings,  spiders 
display  great  acuteness  in  the  development  of  their  sense 
organs.  They  can  perceive  objects  at  a  distance  and  jump 
at  them  with  great  speed  and  accuracy.  They  have  many 
structural  adaptations  which  enable  them  to  vary  their 
activities  with  changing  conditions  of  life.  But  their 
mental  powers  are  not  highly  developed.  Their  usual 
activities  are  simple  movements  or  series  of  movements, 
which  are  repeated  over  and  over  if  a  certain  set  of  condi- 
tions is  repeated.  Adjustments  involving  mental  processes 
are  simple.  Peckham  tells  of  a  spider  in  his  garden  which 
dropped  off  its  web  when  a  tuning  fork  was  sounded.  It 
fell  to  the  ground  five  or  six  times  each  day  and  then  re- 
sponded no  more  to  the  sound.  It  kept  this  up  for  fifteen 
days  and  then  ceased  to  respond  at  all.  Such  behavior 
shows  some  ability  to  learn  and  remember.  The  processes 
involved  are  very  simple,  however,  and  do  not  allow  us  to 
assume  that  the  mental  powers  of  the  spider  rank  very 
high  among  those  of  animals  in  general. 

There  are  a  number  of  different  kinds  of  spiders  and 
many  differ  greatly  from  the  orb-weavers  in  their  habits. 
The  following  are  among  the  most  important  families: 
Thomisidce,  crab-spiders,  spin  no  webs,  but  lurk  in  flowers 


130  GENERAL  ZOOLOGY 

to  capture  their  prey;  many  are  colored  so  as  to  closely 
resemble  the  flowers  in  which  they  usually  hide.  The 
Attidce,  or  jumping  spiders,  are  very  stocky.  They  do  not 
spin  webs  but  hunt  over  the  surfaces  of  plants,  buildings, 
etc.  The  Drassidce,  ground-spiders,  and  Lycosidce,  running- 
spiders,  also  spin  very  little,  but  seek  small  animals  on  the 
ground  or  among  vegetation.  Among  the  web-makers 
there  are  four  important  families.  The  Agelenidce  are 
the  tube-weavers  or  tent-weavers,  which  make  flat  snares 
in  the  grass  (Fig.  59).  There  is  usually  a  little  tube  at 
one  side  in  which  the  spider  lurks.  The  Lyniphiadce 
build  flat  irregular  webs;  the  Epeiridce  make  round  orbs; 
and  the  Therididce  build  irregular  webs  in  the  spaces  among 
vegetation. 

ORDER  2.     SCORPIONIDEA 

Scorpions  (Fig.  60)  are  confined  to  the  warmer  parts  of 
the  earth.  They  are  readily  distinguished  from  other 
arachnids  by  the  character  of  the  abdomen,  which  is  very 
slender  behind  and  bears  a  poisonous  sting  at  its  tip.  The 
sting  is  used  for  paralyzing  small  animals  captured  for 
food,  but  may  also  serve  as  a  weapon  of  defense.  Scor- 
pions breathe  through  lung  books,  like  spiders.  They  have 
many  curious  mating  habits.  Some  species  dance;  in 
many  the  females  carry  their  young  about  on  their  backs. 
When  a  scorpion  stings  a  human  being  there  are  usually  no 
serious  consequences.  The  resulting  irritation,  pain,  and 
swelling  as  a  rule  cease  in  a  day  or  two,  but  in  some  in- 
stances even  death  may  result.  Stings  are  more  serious 
for  children  than  adults. 

ORDER  3.     PHALANGIDA 

The  phalangids  (Fig.  60)  are  commonly  known  as 
" daddy-long-legs "  or  " harvest-men.7'  They  always  have 
very  long  slender  legs,  the  abdomen  is  distinctly  segmented, 
and  there  is  no  slender  "  waist"  between  the  cephalothorax 


ARACHNIDA  131 

and  abdomen,  as  in  true  spiders.  Phalangids  are  nocturnal. 
They  lurk  in  quiet  nooks  during  the  day  and  come  out  at 
night  to  feed  on  small  insects  and  other  animals.  Their 
food  habits  make  them  beneficial  to  man. 


Fia.  60. — Scorpion  carrying  young  and  killing  a  spider;  phalangid  eating  a  fly. 

ORDER  4.    ACARINA 

This  order  includes  the  mites  and  ticks  (Fig.  61).  These 
arachnids  are  wholly  without  external  evidences  of  seg- 
mentation. The  body  is  not  even  divided  into  regions, 
the  head,  thorax,  and  abdomen  being  fused  into  one  mass. 
Many  of  the  ticks  and  mites  are  parasitic  in  their  .habits 
and  some  carry  diseases.  All  are  able  to  go  for  long  periods 
of  time  without  food,  sometimes  for  as  much  as  three  or 
four  years. 

The  mites  are  all  of  small  size  and  their  bodies  are  never 
greatly  swollen.  There  are  several  important  species  in 
the  United  States.  The  poultry  mite,  Dermanyssus  gallince 
sucks  the  blood  of  fowls  and  annually  causes  great  losses. 
The  chiggers,  or  harvest  mites,  Trombididce,  are  common 
on  low  vegetation  and  hence  often  get  on  men  working  in 
fields.  As  six-legged  larvae,  they  commonly  attack  the 
skin  and  cause  severe  irritation.  Preventative  measures 
consist  in  sprinkling  powdered  sulphur  or  naphelene  in  the 
underclothing.  Bites  are  treated  with  ammonia  or  weak 
carbolic  solution.  The  disease  known  as  scabies  or  seven- 


132 


GENERAL  ZOOLOGY 


year  itch,  is  caused  by  a  mite,  Sarcoptes  scabei,  which  lives 
in  little  tunnels  in  the  skin.  Another  mite,  Demodex 
folliculorum,  commonly  lives  in  the  oil  glands  of  the  skin, 
particularly  those  on  the  nose  and  face,  but  apparently 
does  no  harm.  The  scab-parasite,  Psoroptes  communis, 
attacks  sheep,  cattle,  and  horses,  causing  sores  on  the 
skin. 


FIG.  61. — Mites  and  ticks.     A,  B,  nymph  and  adult  of  chigger,  or  harvest  mite; 
C,  the  follicle  mite,  Demodex;  D-G,  ticks:  male,  female,  larva  and  nymph. 

Ticks  are  larger  than  mites  and  adult  females  become 
greatly  swollen,  some  reaching  the  size  of  a  lima  bean. 
The  cattle  tick,  Margaropus  annulatus,  is  the  best  known 
representative  in  this  country.  An  adult  female  of  this 
species,  after  filling  herself  with  blood,  drops  to  the  ground, 
and  lays  several  thousands  of  eggs.  As  soon  as  the  six- 
legged  "seed  ticks"  hatch,  they  climb  up  to  the  top  of  the 
nearest  grass  and  await  the  passing  of  some  animal  which 


ARACHNIDA  133 

may  serve  as  a  host.  If  need  be,  they  remain  thus  on  the 
alert  for  several  months,  patiently  Baiting  through  wind, 
sun,  and  rain.  If  the  larva  succeeds  in  attaching  itself  to 
a  cow  or  some  other  animal,  it  feeds,  and  then  moults  to 
become  an  eight-legged  nymph.  The  nymph  takes  another 
meal  and  sheds  its  skin  to  emerge  as  an  adult  tick.  There 
are  a  number  of  species  of  ticks  in  the  United  States. 
They  attack  man,  poultry,  and  other  animals. 

Though  ticks  may  cause  some  trouble  and  inconvenience 
by  sucking  blood,  their  chief  claim  to  economic  importance 
lies  in  their  ability  to  transmit  diseases.  The  cattle  tick 
carries  Texas  fever.  This  disease  is  caused  by  a  proto- 
zoan parasite,  Babesia  bigemina,  which  not  only  flourishes 
inside  a  tick  in  the  blood  sucked  from  an  infected  animal, 
but  may  also  be  transmitted  by  a  female  tick  to  her  off- 
spring. The  national  government  has  recently  worked 
out  methods  for  the  eradication  of  Texas  fever.  One  of 
the  best  is  the  rotation  of  cattle  through  several  pastures 
so  as  to  free  them  from  ticks.  The  disease  has  caused  losses 
amounting  to  $100,000,000  in  a  single  year  in  the  United 
States.  Another  tick,  Dermacentor  andersoni,  is  prevalent 
in  certain  parts  of  the  northwestern  United  States.  It 
carries  and  transmits  the  spotted  fever,  or  mountain  fever, 
a  fatal  disease.  In  West  Africa  ticks  are  very  numer- 
ous and  carry  most  of  the  prevalent  fevers  affecting  man 
and  other  animals.  Several  cases  have  been  recorded  in 
America  in  which  paralysis  occurred  in  persons  bitten  by 
ticks. 


CHAPTER  XII 
PHYLUM  PROTOZOA 

Protozoa  are  single-celled  animals.  In  many  species 
great  numbers  of  cells  are  grouped  together  to  form  colonies, 
but  in  such  cases  there  is  no  division  of  labor  between  the 
cells;  tissues  are  not  formed,  as  in  true  many-celled  animals. 
A  protozoon,  then,  has  all  the  characteristics  of  single  cells 
as  described  in  Chapter  IV.  Each  protozoon,  though  con- 
sisting of  but  a  single  cell,  is  able  to  feed,  escape  dangers, 
reproduce,  and  carry  on  all  the  activities  characteristic  of 
living  things  (Chapter  III).  Most  protozoans  are  very 
small  and  can  usually  not  be  seen  without  the  aid  of  a 
microscope.  But  what  they  lack  in  size  is  made  up  in  num- 
bers, and  there  is  no  bit  of  natural  water  or  soil  without 
them.  They  often  swarm  by  myriads  in  fields,  lakes,  rivers, 
puddles,  eave  troughs,  cisterns,  water  pipes,  and  other  wet 
places.  Many  species  live  as  accidental  guests  or  as  para- 
sites in  the  bodies  of  many-celled  animals,  some  even  caus- 
ing important  diseases  of  man  and  the  domestic  animals. 

There  are  four  classes  of  Protozoa,  arranged  primarily 
according  to  methods  of  locomotion.  They  are  as  follows: 

1.  Sarcodina    (Rhizopoda).     Protozoa   in    which   locomotion   is   by 
means  of  rather  blunt  protoplasmic  projections  called  pseudopodia. 

2.  Mastigophophora.     Protozoa   which   have   whip-like  flagella   for 
locomotion. 

3.  Sporozoa.    Parasitic  protozoans  possessing  no  special  locomotor 
organs,  and  in  which  there  is  at  times  a  formation  of  many  spores  for 
reproductive  purposes. 

4.  Infusoria.     Protozoa  which  have  the  body  covered  with  many 
hair-like  cilia  for  locomotion. 

134 


PROTOZOA  135 

CLASS  I.     SARCODINA 

The  Sarcodina  are  naked  cells  (Fig.  62,  A-D).  Many 
have  shells  secreted  around  their  little  bodies,  but  such 
accretions  are  not  actually  a  part  of  the  protoplasm.  The 
protoplasm  of  these  animals  is,  then,  without  much  speciali- 
zation, but  it  may  form  elaborate  structures  on  the  outside. 
The  amoeba  will  be  considered  as  a  representative  of  the 
Sarcodina. 

THE  AMCEBA 

Self-maintenance. — There  are  a  number  of  different 
kinds  of  amoebas  and  their  structure  and  habitats  vary 
considerably.  The  species  chosen  for  the  subject  of  this 
discussion,  Amoeba  proteus  (Fig.  62,  A),  is  found  in  water,  in 
the  brown  coating  of  microscopic  plants  such  as  accumulates 
in  water  troughs  and  among  aquatic  vegetation.  Its  food 
consists  of  minute  plants  and  animals  which  are  engulfed 
by  the  soft  body.  There  is  no  mouth  or  definite  place  for 
the  entrance  of  food.  The  body  consists,  like  all  cells,  of  a 
central  nucleus  surrounded  by  cytoplasm.  The  cytoplasm 
is  of  about  the  consistency  of  the  white  of  an  egg,  and  shows 
a  differentiation  into  two  regions:  an  outer  denser  portion, 
the  ectosarc,  and  an  inner  granular  portion,  the  endosarc. 
When  an  amoeba  engulfs  its  food  the  ectosarc  is  first  turned 
in  to  form  a  little  pocket,  then  surrounds  the  food  com- 
pletely. The  obj  ect  eaten  goes  into  the  endosarc,  surrounded 
by  a  little  water  and  the  ectosarc  which  came  in  as  it  was 
being  engulfed.  The  watery  secretion  formed  around  the 
particle  makes  a  "food  vacuole,"  and  contains  certain  sub- 
stances which  dissolve  the  food  so  that  it  may  be  gradually 
absorbed  by  the  surrounding  endosarc.  If  there  are  sub- 
stances in  the  food  which  cannot  be  digested  and  assimi- 
lated, they  are  dropped  out  from  the  body  anywhere,  as  the 
amoeba  moves  along.  In  spite  of  the  extreme  simplicity  of 
the  amoeba's  body,  it  is  able  to  discriminate  between  objects 
fit  for  food  and  those  which  are  unsuitable.  It  rejects  sand 


136 


GENERAL  ZOOLOGY 


grains  or  other  worthless  particles,  but  persistently  follows 
moving  animals  or  plants  which  may  serve  for  food.  Such 
behavior  shows  that  the  amoeba  has  a  simple  "  chemical 
sense/'  or  what  corresponds  to  taste  and  smell  in  higher 
animals. 

After  food  has  been  absorbed,  it  flows  about  in  the  endo- 
plasm  so  that  all  parts  are  supplied  with  nourishment. 


FIG.  62. — Protozoa.  A-D,  Class  Sarcodinia;  E-G,  Class  Mastigophora; 
H-K,  Class  Infusoria.  A,  Amoeba;  B,  Arcella;  C,  Diffiugia;  D,  Actinosphoerium; 
E,  Mastigamoeba;  F,  Euglena,  G,  Volvox;  H,  Paramcecium;  I,  Vorticella;  J, 
Stentor;  K,  Stylonychia. 

The  amceba  is  less  than  a  thousandth  of  an  inch  in  length 
and  therefore  does  not  need  special  adaptations  for  respira- 
tion. Waste  gases  are  shed  into  the  surrounding  water  and 
oxygen  is  readily  absorbed.  Waste  products  resulting 
from  metabolism  are  dissolved  in  the  water  fluids  within 
the  body  and  flow  into  a  little  reservoir,  the  contractile 


PROTOZOA  137 

vacuole,  which  pulsates  regularly  and  empties  its  accumu- 
lated wastes  through  several  pores  opening  through  the 
ectosarc. 

Self -protection. — The  amoeba  is  so  small  that  it  has  few 
enemies .  It  could  no t  run  away  very  successfully  if  pursued, 
for  it  moves  very  slowly  by  projecting  little  soft  finger-like 
pseudopods  on  which  it  walks  or  flows  along.  Sometimes 
one  amceba  will  chase  another  and  eat  it. 

The  worst  dangers  for  amcebas,  however,  are  the  changes 
in  physical  conditions  which  may  come  with  the  drying  up 
of  the  water  in  which  they  live,  or  with  the  changing  seasons. 
If  such  unfavorable  changes  do  not  come  too  suddenly  an 
amoeba  secretes  a  resistant  covering  about  its  body  and 
remains  in  a  quiescent  state,  biding  its  time  until  conditions 
are  again  favorable.  Such  a  resting  amceba  is  said  to  be 
encysted,  and  is  called  an  amceba  cyst. 

The  amceba  lives  from  day  to  day  so  as  to  escape  ordi- 
nary dangers  as  far  as  possible.  It  is  wholly  without  any- 
thing which  may  be  interpreted  as  an  eye,  yet  is  sensitive  to 
light.  This  sensitiveness  is  of  use  to  the  amceba  in  enabling 
it  to  avoid  the  direct  rays  of  the  sun,  which  would  be  injuri- 
ous, and  allows  it  always  to  creep  away  from  the  light  into 
dark  crevices  where  it  finds  food  and  shelter.  The  amceba 
is  therefore  protected  from  harm  because  it  seldom  comes 
out  into  the  open. 

Race  Preservation. — For  an  amoeba,  the  formation  of 
new  individuals  is  an  easy  matter.  One  individual  has  only 
to  undergo  cell-division  to  produce  two,  and  if  each  of  the 
" offspring"  produced  in  this  way  continues  to  divide  at  in- 
tervals, the  final  total  will  be  enormous.  When  an  amoeba 
divides,  the  body  forms  two  cells  of  equal  size  and  hence 
the  process  is  known  as  binary  fission.  Such  fission  may 
continue  to  be  the  only  means  of  multiplication  for  several 
hundreds  of  generations,  but  if  continued  too  long  the  last 
generations  lack  vitality  and  the  stock  finally  dies  out.  An 
amceba,  therefore,  does  not  pass  through  its  life  cycle  as  an 
individual.  Starting  with  a  "young"  individual  it  divides 


138  GENERAL  ZOOLOGY 

in  the  succeeding  generations  to  form  other  amoebas  and 
after  a  time  maturity,  old  age,  and  death  are  attained. 
Any  one  individual  is  young,  or  mature,  or  old,  through  all 
its  "  life." 

If  binary  fission  were  the  only  process  concerned  in  the 
reproduction  of  amoebas,  they  would  probably  all  have  died 
of  old  age  long  ago.  But  there  is  another  way  in  which  in- 
dividuals may  be  produced.  This  is  by  conjugation.  At 
intervals  two  amoebas  come  together  and,  after  a  prepara- 
tory or  ripening  process,  exchange  some  nuclear  material. 
When  they  separate  again  they  are  not  the  same,  for  each 
contains  part  of  the  other.  There  is  also  another  and  more 
important  change  resulting  from  conjugation.  After  it  is 
completed  each  of  the  conjugants  is  young,  no  matter  what 
its  condition  was  before.  An  amoeba  may  be  mature  or  in 
the  last  stages  of  old  age,  and  after  conjugation  be  rejuve- 
nated, starting  anew  with  all  the  vigor  of  youth. 

When  compared  with  other  rhizopods  (Fig.  62),  the 
amoeba  shows  great  simplicity  of  structure.  It  is  a 
bit  of  naked  protoplasm  with  a  nucleus  and  a  contrac- 
tile vacuole.  Some  of  its  relatives  have  little  shells  about 
the  body.  Arcella  (B)  has  a  hemispherical  shell  composed 
of  chitin;  Difflugia  (C)  encloses  itself  by  sticking  sand 
granules  together  to  make  a  beautiful  little  case.  In  one 
order  of  rhizopods,  the  Foraminifera,  a  calcareous  shell 
is  secreted  within  the  cytoplasm  of  the  cell.  These  shells 
are  of  various  forms  in  different  species — like  rods,  wheels, 
strings  of  beads,  etc.  The  largest  protozoan  known  is  a 
fossil  foraminifer,  Nummulites,  which  is  disc-shaped  and 
sometimes'  has  a  diameter  of  about  two  inches.  Chalk 
deposits  like  those  at  Austin,  Texas,  are  made  up  mostly 
of  the  shells  of  Foraminifera.  The  rhizopods  in  the  order 
Heliozoa  (D)  do  not  have  a  shell  but  possess  very  slender 
pseudopodia  which  radiate  from  the  body  and  have  per- 
manent rod-like  supports  in  them.  The  Radiolaria  are 
marine  rhizopods  which  have  slender  pseudopodia  like 


PROTOZOA  139 

those  of  the  Heliozoa,  and  also  have  elaborate  basket-like 
internal  shells  of  silica. 

Some  amcebas  cause  important  diseases  of  man  and  other 
animals.  Entamceba  histolytica  burrows  in  the  wall  of  the 
intestine,  swallows  red  blood  corpuscles,  and  is  the  cause 
of  dysentery ;  Entamceba  gengevalis  is  associated  with  pyhor- 
rhea  alveolaris,  a  very  prevalent  disease  of  the  teeth. 

CLASS  II.     MASTIGOPHORA 

Mastigophorans  (Fig.  62,  E-G)  all  have  one  or  more 
flagella  for  locomotion,  and  on  this  account  are  commonly 
known  as  flagellates.  A  flagellum  is  a.  slender  hair-like 
process  which  pulls  the  cell  along  by  lashing  about.  In 
addition  to  this  locomotor  appendage,  the  Mastigophora 
are  generally  to  be  distinguished  from  the  Rhizopoda  by 
the  presence  of  a  membrane,  or  pellicle.  This  is  a  thin 
outer  covering  which,  though  it  may  be  pliable,  gives  the 
body  a  more  definite  form.  The  simplest  mastigophorans, 
however,  lack  such  a  covering.  Mastigamceba  (Fig.  62,  E), 
for  example,  is  much  like  an  amceba  with  a  single  long 
flagellum.  Its  body  is  naked,  and  it  can  crawl  about  by 
protruding  pseudopodia,  but  it  may  also  swim  actively 
through  the  water  by  moving  its  flagellum. 

Many  mastigophorans  show  relationships  to  plants.  All 
green  plants  contain  chlorophyll,  a  substance  which  is 
able  in  the  presence  of  sunlight  to  elaborate  organic  com- 
pounds (starch,  etc.)  from  simple  substances  like  carbon 
dioxide  and  water.  Such  "  manufacturing "  is  known  as 
photosynthesis.  Many  of  the  protozoans  in  this  class 
contain  the  green  chlorophyll  and  are  able  to  carry  on 
photosynthesis  like  that  in  plants.  Another  indication  of 
relationship  between  mastigophorans  and  plants  is  the  fact 
that  the  motile  stages  (swarm  spores)  of  a  number  of  the 
simpler  plants  are  very  much  like  free  living  mastigophorans 
which  do  not  come  from  plants.  We  have  at  one  extreme, 
mastigophorans  which  are  clearly  animals,  and  at  the  other, 


140  GENERAL  ZOOLOGY 

flagellated  organisms  which  are  clearly  plants.  On  account 
of  such  mixed  relationships  some  biologists  make  a  group, 
Flagellata,  which  contains  both  plants  and  animals,  and 
which  is  believed  to  have  given  rise  to  plants  by  emphasizing 
certain  qualities,  and  to  animals  by  stressing  others.  Ac- 
cording to  this  view  the  flagellates  were  the  first  living 
things  and  hence  stand  at  the  base  of  the  evolutionary 
series.  A  form  like  the  Amceba  may,  therefore,  not  be  primi- 
tive, even  though  it  is  simpler  in  structure  than  any  other 
animal,  but  perhaps  may  have  arisen  from  the  more  complex 
flagellates  by  the  loss  of  some  structures.  Mastigamceba 
in  such  an  evolutionary  history  might  serve  as  a  "  missing 
link/' 

Some  flagellates  almost  become  metazoans.  There  are 
many  species  in  which  colonies  are  formed  by  cells  remaining 
united  after  fission.  In  such  colonies  the  number  of  cells 
may  be  variable  or  very  constant.  In  certain  species  there 
are  always  four,  eight,  sixteen,  or  thirty-two  cells.  Others 
have  a  very  large  number;  a  colony  of  Volvox  (Fig.  62,  G), 
for  example,  may  be  made  of  12,000  cells.  In  colonies  of 
flagellates  there  is  often  a  division  of  labor  between  the  cells 
—some  carry  on  locomotion  and  feeding,  others  serve  for 
reproduction  only.  There  is  thus  a  differentiation  into 
somatic  (body)  and  germ  (reproductive)  cells.  In  Volvox 
and  some  other  colonies  there  is  a  further  division  of  labor 
in  the  germ  cells;  some  developing  into  large  ova  (eggs)  and 
others  into  small  sperm  cells.  This,  however,  is  the  limit 
of  cell-specialization  in  flagellates,  or  in  protozoans,  and  the 
colonies  cannot  properly  be  classed  as  many-celled  animals 
because  the  somatic  cells  are  all  alike — i.e.,  there  are  no 
tissues. 

There  are  a  number  of  flagellates  which  cause  serious 
diseases.  Different  species  of  the  genus  Trypanosoma 
cause  sleeping  sickness  in  man,  surra  and  dourine  in  horses, 
nagana  in  cattle,  etc.  The  Texas  fever  which  is  prevalent 
in  cattle  throughout  the  southern  United  States  is  caused 


PROTOZOA  141 

by  a  flagellate.     There  are  several  species  which  live  in  the 
alimentary  canal  of  man  and  other  animals. 

CLASS  III.     SPOROZOA 

This  group  of  protozoans  contains  only  species  which  are 
entirely  parasitic,  and  they  may  live  in  representatives  of 
any  phylum  of  the  animal  kingdom.  There  are  some  which 
even  dwell  in  the  bodies  of  other  protozoans.  As  would  be 
expected  from  their  parasitic  habits,  many  sporozoans 
cause  diseases  in  the  animals  they  inhabit.  The  sporozoan 
disease  of  most  interest  to  man  is  malarial  fever,  prevalent 
in  all  the  warm  regions  of  the  earth. 


1.  Day 


Fever  day 
2.  Day 


3.  Day 


Fever  e/atf 
4.  Day 


\ 


\ 


I 


\ 


A  B  C  D          A(  B'          C'  D'         /I" 

FIG.  63. — A  temperature  chart  of  a  man  sick  with  malarial  fever,  and  (below) 
red  blood  corpuscles  containing  the  parasites  causing  the  disease.  Note  that 
there  is  high  fever  when  an  infected  corpuscle  ruptures,  thus  freeing  spores, 
poison,  and  black  pigment.  A,  A',  A",  young  Plasmodium  in  corpuscle;  B,  B', 
well  grown;  C,  C',  nearly  mature;  D,  D',  ripe  spores  escaping. 

Malaria  is  caused  by  parasites  of  the  genus  Plasmodium 
and  there  are  three  different  species  which  cause  character- 
istic fevers.  Plasmodium  vivax  causes  "  benign  tertian 
malaria"  (i.e.,  mild  malarial  fever  in  which  there  is  fever 
every  third  day).  This  parasite  can  enter  a  man  only  when 
a  certain  mosquito  bearing  the  disease  bites  him,  and  it  then 
migrates  from  the  insect's  proboscis  directly  into  the  blood. 
Once  in  the  blood  it  enters  a  red  corpuscle  and  grows  for 


142  GENERAL  ZOOLOGY 

about  a  day  and  a  half,  then  divides  into  several  little 
spores  (Fig.  63).  The  infected  corpuscle  finally  bursts  open, 
setting  the  spores  and  a  poison  free  in  the  blood.  The 
poison  causes  the  febrile  condition  which  a  patient  shows. 
The  little  spores  enter  new  corpuscles,  grow  for  two  days, 
and  then  release  another  crop  of  spores.  There  is  thus  a 
periodic  recurrence  of  fever,  every  third  day,  which  is  corre- 
lated with  a  certain  stage  in  the  development  of  the  parasite. 
;  Three  things  are  necessary  before  malaria  can  become 
established  in  a  region:  (1)  infected  people,  (2)  anophelene 
mosquitoes,  and  (3)  warm  climate.  A  man  may  carry  the 
parasites  and  serve  as  a  " reservoir"  long  after  he  has 
ceased  to  have  fever.  The  mosquito  cannot  transmit  the 
disease  as  soon  as  it  draws  blood  from  an  infected  person, 
but  must  " incubate"  the  parasite  for  about  twelve  days. 
During  this  incubation  period  the  germ  cells  of  the  parasite 
conjugate  and  young  spores  are  produced  which  take  up 
their  position  in  the  mosquito's  salivary  glands,  ready  to 
enter  the  next  man  who  is  bitten. 

CLASS  IV.     INFUSORIA 

In  this  class  of  protozoans  cilia  are  the  characteristic 
organs  of  locomotion  (Fig.  62,  H-K).  Cilia  are  much  like 
flagella,  but  are  smaller  and  always  occur  together  in  large 
numbers.  There  is  always  a  cell-membrane  or  pellicle 
covering  the  body  of  a  ciliate  and  two  nuclei  are  usually 
present.  The  larger,  or  trophonucleus,  is  concerned  with 
ordinary  metabolic  activities;  the  smaller  micronucleus  is 
usually  active  only  during  fission  or  conjugation.  The  cells 
of  some  ciliates  are  the  most  complicated  in  the  animal 
kingdom.  It  will  be  well,  therefore,  to  consider  the  activi- 
ties of  a  typical  representative  in  some  detail  in  order  that  it 
may  be  compared  with  the  amoeba  (page  135),  which  is  one 
of  the  simplest  of  "cells.  For  this  purpose  the  "bell  animal- 
cule" Vorticella,  is  chosen. 


PROTOZOA  143 

Vorticelia  Campanula  Ehrenberg 

Self -maintenance. — Vorticelia  (Fig.  62,  I)  is  to  be  found 
in  fresh- water  ponds  and  ditches  attached  to  some  object  by 
a  slender  stalk.  The  specific  name,  campanula,  refers  to  the 
shape  of  the  body,  which  resembles  a  dinner-bell  with  a  very 
long  handle.  Around  the  distal  margin  of  the  bell  there  is  a 
row  of  cilia.  These  by  their  continual  beating  create  a 
vorfcex  in  which  the  water  is  drawn  toward  the  mouth — 
hence  the  generic  name,  Vorticelia. 

This  little  protozoan  feeds  on  minute  particles  brought 
to  it  in  the  currents  created  by  the  cilia.  If  unsuccessful 
when  drawing  water  from  one  source,  it  jerks  back  by  con- 
tracting and  coiling  the  stalk,  and  then  stretches  out  in  a 
new  direction,  thus  increasing  its  chances  of  finding  organ- 
isms on  which  it  may  feed.  There  are  little  contractile 
strands  passing  from  one  end  of  the  stalk  to  the  other,  and 
these  are  just  as  much  specialized  for  contraction  as  the 
fibrils  in  the  muscle  cells  of  a  vertebrate.  Food  particles 
enter  the  mouth  after  passing  through  a  short  gullet,  and  are 
enclosed  in  a  little  food  vacuole  before  beginning  to  circulate 
in  the  endoplasm.  Undigested  food  passes  out  through  a 
definite  opening  in  the  pellicle.  There  is  a  contractile 
vacuole  near  the  distal  end  of  the  bell,  which  serves  for  ex- 
creting metabolic  waste  products.  The  trophonucleus  in 
Vorticelia  is  large  and  U-shaped.  It  thus  extends  through 
a  considerable  part  of  the  interior  and  can  readily  control 
metabolism. 

Self -protection. — Vorticelia  has  few  enemies.  There  are 
some  sporozoans  which  live  within  it  as  internal  parasites. 
Some  protozoans  and  larger  animals  feed  upon  it.  It  cannot 
run  away  when  set  upon;  its  only  method  of  escape  is  to 
shorten  the  stalk  as  much  as  possible  and  remain  quiet  until 
danger  is  past. 

If  the  conditions  of  life  are  unfavorable  in  a  certain  place, 
a  Vorticelia  may  break  loose  from  its  stalk  and  swim  to  some 
better  spot,  where  it  forms  an  attachment  and  grows  a  new 


144  GENERAL  ZOOLOGY 

stalk.  If  conditions  become  unfavorable  everywhere,  a 
wall  is  formed  around  the  body,  and  encystment  takes  place. 
Vorticella  can  then  remain  in  a  quiescent  condition  until 
better  times  come. 

Race  Preservation. — Vorticella  can  produce  new  individu- 
als by  fission.  The  trophonuclei  and  micronuclei  elongate 
and  divide,  the  bell  becomes  wide,  and  a  constriction  ap- 
pears at  the  distal  end  which  splits  the  cell  longitudinally 
into  two  parts,  each  with  a  pair  of  nuclei.  Two  individuals 
are  thus  formed,  attached  to  one  stalk.  One  of  them  soon 
forms  a  second  band  of  cilia  about  its  body,  breaks  loose  and 
swims  away  to  settle  down  and  form  a  stalk  of  its  own.  Vor- 
ticella may  multiply  for  many  generations  by  such  binary 
fission  but  will  finally  die  of  old  age  unless  conjugation  takes 
place. 

For  conjugation  two  kinds  of  individuals  are  formed, 
macrogametes  and  microgametes.  The  former  are  stalked 
individuals  differing  neither  in  size  nor  in  any  other  obvious 
feature  from  an  ordinary  Vorticella.  The  microgametes, 
as  their  name  implies,  are  much  smaller.  They  are  also 
peculiar  in  possessing  a  posterior  circle  of  cilia  about  the 
body  and  in  being  without  a  stalk.  A  microgamete  when 
set  free  from  the  "parent"  (by  cell-division),  swims  about 
until  it  finds  a  macrogamete  and  then  conjugates.  Con- 
jugation begins  at  daybreak  and  lasts  thirteen  or  fourteen 
hours.  The  cytoplasm  of  the  macrogamete  fuses  with  that 
of  the  microgamete.  The  trophonuclei  in  both  cells  then 
degenerate  and  take  no  part  in  what  follows.  The  micro- 
nucleus  in  each  cell,  after  dividing  several  times  in  order  to 
cast  out  some  of  the  nuclear  material,  is  ripe,  and  then  fuses 
with  a  nucleus  formed  from  the  micronucleus  originally  in 
the  other  cell.  Thus  a  single  new  cell,  the  zygote,  origi- 
nates from  the  macrogamete  and  the  microgamete,  which 
were  two  separate  cells  at  the  beginning  of  conjugation. 

When  compared  with  those  in  the  amoeba,  the  conju- 
gating cells  in  Vorticella  show  progress  along  the  line  of  sex 
differentiation.  The  small  motile  microgamete  has  male 


PROTOZOA  145 

characteristics;  the  larger  stationary  macrogamete  is  the 
female.  Vorticella  is  not  as  specialized  in  this  particular 
as  some  of  the  flagellates  and  sporozoans  (Volvox,  page  140; 
Plasmodium,  page  141). 

GENERAL    FACTS    ABOUT    PROTOZOA 

Protozoa  are  remarkable  among  animals  because  their 
entire  bodies  consist  of  single  cells.  Some  of  them  are  the 
most  complex  animal  cells  known,  others  are  the  most 
simple.  Metabolism  in  many  protozoans  is  extremely  rapid. 
One  species,  Didinium  nasutum,  can  devour  other  proto- 
zoans that  are  ten  times  its  own  size,  the  body  becoming 
stretched  so  that  it  forms  a  thin  covering  for  the  food,  yet  in 
an  hour  or  two  it  is  ready  to  eat  another  meal  of  similar 
size. 

It  is  often  said  that  "the  Protozoa  are  immortal,"  and 
this  may  indeed  be  true  of  most  species.  When  a  protozoan 
cell  is  about  to  die  of  old  age,  it  may  become  young  again  by 
conjugation  or  by  fusing  with  another  cell,  but  must,  of 
course,  sacrifice  its  identity  by  so  doing.  This  is  not  true, 
however,  of  protozoans  in  which  there  is  a  differentiation 
into  somatic  (body)  and  germ  (reproductive)  cells,  (Volvox, 
Plasmodium).  In  these,  as  in  many-celled  animals,  the 
"body"  must  die  and  only  the  germ  cells  may  help  to  form 
a  new  generation.  The  statements  in  regard  to  the  necessity 
for  conjugation  in  order  to  attain  youth  would  probably  not 
be  accepted  by  Professor  L.  L.  Woodruff,  who  has  reared  a 
ciliate  (Paramcecium)  for  more  than  five  thousand  genera- 
tions without  conjugation.  In  a  recent  paper  (1916)  he 
says:  "an  individual  of  Paramoecium  is  self-sufficient  to 
reproduce  indefinitely  without  recourse  to  conjugation." 
If  Woodruff's  point  of  view  is  accepted,  it  may  be  necessary 
to  revise  completely  the  current  ideas  in  regard  to  the  pro- 
duction of  youth,  or  it  may  be  found  that  only  a  few  proto- 
zoans are  able  to  multiply  indefinitely  by  fission  without 
dying  of  old  age.  Perhaps  the  latter  alternative  may  be 
looked  upon  as  the  more  probable  for  among  many-celled 


146  GENERAL  ZOOLOGY 

animals  there  are  only  a  very  few  cases  of  parthenogenesis, 
or  development  of  ova  without  fertilization,  and  consider- 
ing the  whole  animal  kingdom  there  are  very  few  cases  in 
which  new  individuals  do  not  originate  from  zygotes. 
Another  view  regarding  the  effects  of  conjugation  has  been 
advocated  by  a  number  of  zoologists,  i.e.,  " it  is  more  prob- 
able that  conjugation  results  in  increased  powers  of  adapta- 
bility than  in  rejuvenescence." 

If  protozoans  are  always  single  cells,  it  is  but  natural  to 
ask  what  their  psychic  capabilities  may  be.  Can  they 
profit  by  experience?  Have  they  memory?  In  connection 
with  the  discussion  of  Amoeba  and  Vorticella  it  has  already 
been  shown  that  protozoans  can  vary  their  behavior  some- 
what to  meet  changes  in  surrounding  conditions,  but  for 
the  most  part  their  activities  are  simple  and  are  repeated 
over  and  over  if  stimulating  conditions  are  repeated.  A 
free-swimming  individual  will  start  in  a  certain  direction 
and  keep  going  until  it  finds  food  (or  a  mate,  if  the  breed- 
ing instincts  are  active).  If  some  unfavorable  stimulus  is 
encountered,  however,  it  backs  up  a  little  and  starts  off 
in  a  new  direction.  If  conditions  are  again  unfavorable, 
there  is  more  backing  and  trying,  until  a  proper  course  is 
found.  The  life  of  an  active  protozoan,  then,  is  a  series 
of  continual  testings  of  the  surroundings ;  success  is  attained 
by  holding  to  what  promises  to  be  good  and  avoiding  what 
appears  to  be  bad.  There  is  no  reasoning,  no  planning 
or  forethought. 

The  Protozoa  present  one  other  point  of  general  interest — 
they  were  doubtless  the  ancestors  of  the  Metazoa.  Many 
protozoans  have  almost  become  many-celled  animals,  in 
fact,  it  is  hard  sometimes  to  draw  the  line  between  single 
and  many-celled  forms,  but  none  have  quite  attained 
the  goal.  Most  of  them  have  taken  the  wrong  road  by 
specializing  the  structures  of  single  cells,  instead  of  form- 
ing a  group  of  cells  and  dividing  the  work  to  be  done  among 
them.  Perhaps  many-celled  animals  have  arisen  from  more 
than  one  group  of  protozoans.  The  possible  steps  by  which 
the  Metazoa  arose  will  be  considered  in  the  next  chapter. 


CHAPTER  XIII 

THE  ORIGIN  AND  CHARACTERISTICS  OF  THE 
METAZOA 

ORIGIN   OF   MANY-CELLED   ANIMALS 

There  is  every  reason  to  believe  that  Metazoa  arose  from 
Protozoa;  i.e.,  that  single-celled  animals  gave  rise  to  many- 
celled  animals.  There  is  no  fossil  record  of  such  a  trans- 
formation because  the  earliest  geological  strata  on  the  earth 
have  all  been  modified  so  that  their  records  are  destroyed. 
The  first  records  show  single-celled  animals  living  with 
many-celled  types,  as  they  do  today.  Yet  the  develop- 
ment of  every  metazoan  starts  with  a  single  cell  and  the 
law  of  biogenesis  (Chapter  V),  therefore,  indicates  that  the 
origin  of  Metazoa  was  from  single  cells.  Furthermore, 
all  the  processes  that  take  place  in  the  development  of  a 
metazoan,  up  to  the  formation  of  tissues,  are  duplicated 
in  protozoans.  There  is  a  general  direction  which  all  cell 
associations  take,  and  metazoans  have  simply  gone  farther 
than  protozoans. 

With  these  things  in  mind,  it  is  possible  to  give  a  synop- 
sis of  the  probable  steps  in  the  origin  of  many-celled  animals. 
First,  groups  of  cells  arose  as  a  result  of  the  ordinary  multi- 
plicative processes  in  protozoans — binary  fission.  The  cells 
after  dividing  did  not  separate  but  remained  united.  After 
a  time  such  behavior  became  the  regular  procedure  in  cer- 
tain strains  of  protozoans  and  colonies  of  cells  always  re- 
sulted from  cell-division.  This  gave  opportunity  for  the 
next  step — division  of  labor  between  the  cells.  Some  were 
set  aside  to  carry  on  cell  multiplication  and  reproduction, 
others  looked  after  the  nutritive  and  protective  functions. 
This  specialization  was  a  convenience  for  the  whole  colony, 
for  each  individual  cell  had  fewer  things  to  attend  to.  This 

147 


148  GENERAL  ZOOLOGY 

differentiation  into  soma  and  germ  had,  and  still  has,  one 
disadvantage,  however — all  the  somatic  cells  must  die  when 
the  body  is  worn  out. 

After  germ  and  somatic  cells  were  established  another 
improvement  came  in,  by  dividing  the  labor  between  the 
sexual  cells.  As  stated  in  the  last  chapter  a  young  indi- 
vidual is  produced  by  the  fusion  of  two  sexual  cells  to  form  a 
single  cell.  It  is  desirable,  if  two  sex  cells  are  going  to  fuse, 
to  have  them  able  to  move  so  that  they  can  more  readily 
get  together.  It  is  also  desirable  that  they  contain  reserve 
energy,  so  that  development  may  go  on  after  fusion  has 
taken  place,  during  the  time  when  the  usual  activities  are 
more  or  less  disturbed.  Locomotion  and  storage  of  nour- 
ishment are  therefore  two  essential  qualities  in  germ  cells. 
It  is  not  very  feasible,  however,  to  have  a  cell  store  large 
quantities  of  food  and  at  the  same  time  be  able  to  move 
about  readily.  This  difficulty  has  been  met  by  the  develop- 
ment of  two  kinds  of  germ  cells:  (1)  ova,  which  are  large, 
immobile,  and  contain  much  nourishment ;  and  (2)  sperma- 
tozoa, which  are  minute  and  very  active. 

In  a  number  of  groups  of  protozoans  colonies  of  cells 
have  become  differentiated  to  the  extent  described  up  to 
this  point,  but  only  true  metazoans  have  differentiated 
the  somatic  cells  so  that  tissues  are  formed.  Tissues  are, 
therefore,  the  only  criterion  which  will  separate  one  group 
from  the  other.  If,  in  the  future,  some  colony  of  protozoans 
develops  a  division  of  labor  in  its  somatic  cells,  so  that  one 
group  attends  to  one  function  and  another  group  to  some 
other  activity,  we  will  then  be  obliged  to  classify  it  as  a 
many-celled  animal. 

TISSUES  AND  ORGANS 

A  tissue  is  a  group  of  similar  cells  wThich  carries  on  some 
particular  function  in  a  metazoan's  body.  Tissues  have 
developed  chiefly  along  four  lines,  and  each  of  the  chief 
types  has  arisen  by  emphasizing  some  quality  present  in  all 
protoplasm.  The  groups  are  as  follows: 


TISSUES  AND  ORGANS  149 

1.  Epithelial  tissue  consists  of  thin  sheets  of  cells  which 
usually  cover  surfaces.     The  external  covering  of  meta- 
zoans  consists  of  epithelial  tissues  and  their  products;  the 
linings  of  various  internal  cavities  and  the  membranes  which 
connect  different  organs  are  usually  epithelial. 

2.  Muscular   tissue   has   specialized   by   developing   its 
power   of   contractility.     The   simplest   muscular   tissues 
are  made  up  of  elongated  single  cells,  but  the  more  elabor- 
ate types,   like   the   striated   muscles   of  arthropods   and 
vertebrates,  are  extremely  complex  and  have  various  special 
adaptations  for  making  them  contract  more  effectively. 
There  is  one  point  to  remember  in  regard  to  muscular 
tissue — it  can  only  do  work  when  contracting. 

3.  Nervous  tissue  is  composed  of  cells  which  have  greatly 
developed  their  powers  of  conducting  and  modifying  im- 
pulses.    Most  nerve  cells  have  long  fibers  leading  out  from 
the  cell-body  and  these  carry  nervous  impulses  from  place 
to.  place.     Ganglia  are  places  in  nervous  tissues   where 
there  are  more  cells  then  fibers,  and  they  serve  as  "  relay 
stations"  for  modifying  or  originating  impulses.     Nerves 
are  portions  of  nervous  tissue  which  contain  few  or  no  cell- 
bodies,  but  have  many  fibers. 

4.  Sustentative,  or  Connective  tissue,  is  quite  variable  but 
is  always  characterized  by  the  presence  of  a  large  amount  of 
intercellular  substance.     This  material  between  the  cells 
may  be  bone   (in   osseous  tissue),  fibers  (fibrous  tissue), 
cartilage   (cartilaginous  tissue),  plasma  (blood),  or  other 
matter. 

In  the  bodies  of  most  many-celled  animals  division  of 
labor  has  been  carried  beyond  the  formation  of  tissues. 
Usually  organs  are  formed  by  grouping  various  tissues  to- 
gether for  carrying  on  particular  labors.  The  heart,  for 
example,  is  an  organ  for  pumping  blood,  and  is  composed 
of  muscular,  epithelial,  fibrous  and  nervous  tissue.  The 
hand  is  an  organ  for  prehension  and  offense;  it  is  made  up 
of  bone,  cartilage,  muscle,  nerve,  epithelium,  etc.  Organs 
in  the  more  complex  animals  are  in  turn  grouped  into 


150  GENERAL  ZOOLOGY 

great  systems  which  are  chiefly  as  follows:  digestive,  circu- 
latory, respiratory,  excretory,  muscular,  skeletal,  nervous, 
reproductive. 

REPRODUCTION  IN  METAZOA 

It  has  been  said  that  all  Metazoa  start  as  single  cells,  and 
in  general  this  statement  is  true,  but  there  are  many  excep- 
tions in  particular  cases.  In  fact,  it  may  be  said  that  there 
are  two  methods  of  reproduction — sexual  and  asexual. 
The  former  always  involves  the  union  of  an  ovum  and  a 
spermatozoon  to  produce  a  new  and  young  individual. 
Asexual  means  without  sex,  and  there  are  various  ways  in 
which  new  individuals  may  be  formed  without  the  usual 
fusion  of  germ  cells.  Some  sponges  may  be  cut  up  in  pieces 
and  propagated  as  slips,  like  plants;  others  give  off  groups  of 
cells  as  buds,  and  these  grow  into  new  individuals,  still 
others  have  the  bulk  of  the  body  decay  at  certain  seasons, 
but  at  such  times  groups  of  cells  in  the  interior  form  a  re- 
sistant coating,  live  until  they  can  have  opportunity  to  grow 
again,  and  then  form  a  complete  new  sponge.  Various 
polyps  and  worms  are  able  to  produce  new  individuals  by 
budding  off  portions  of  the  body  or  by  breaking  it  in  two. 
In  addition  to  usual  asexual  methods  of  reproduction, 
there  are  among  metazoans  some  modifications  of  sexual 
processes.  Many-celled  animals  show  two  conditions  in 
regard  to  sex — they  may  be  hermaphroditic  or  dioecious. 
In  the  former  case  an  animal  has  both  male  and  female 
organs  in  its  body,  and  in  rare  instances  is  able  to  fertilize 
itself.  It  can  thus  sometimes  produce  offspring  without 
the  presence  of  another  individual.  There  is  another 
modification  of  sexual  reproduction  shown  in  partheno- 
genesis. An  ovum  will  usually  not  develop  unless  it  has 
united  with  a  sperm  cell  but  there  are  a  few  animals  in 
which  such  union  may  not  be  necessary,  and  ova  therefore 
develop  without  fertilization.  However,  in  most  cases, 
perhaps  in  all,  parthenogenetic  reproduction  cannot  go  on 


ONTOGENY  OF  METAZOA  151 

indefinitely,  but  must  give  place  occasional!}'  to  sexual 
processes,  and  the  great  majority  of  animals  cannot  re- 
produce parthenogenetically  at  all. 

In  spite  of  the  various  ways  in  which  individuals  may 
arise,  the  development  of  any  particular  metazoan  from  a 
fertilized  ovum  usually  shows  a  rather  definite  sequence  of 
events,  and  this  series  of  stages  is  practically  the  same  in 
all  the  different  phyla,  except  one.  Before  passing  on  to 
the  phyla  of  metazoans,  it  will  be  proper  therefore  to 
consider  a  typical  developmental  history. 

ONTOGENY  OF  A  TYPICAL  METAZOAN 

1.  Maturation  (Fig.  64,  A,  B). — A  many-celled  animal 
starts  as  a  single  cell,  the  zygote,  formed  by  the  union  of  the 
egg  cell,  or  ovum,  with  a  sperm  cell,  or  spermatozoon.  Be- 
fore a  zygote  can  be  formed,  however,  there  must  be  cer- 
tain preparatory  or  ripening  changes  in  both  the  egg  and 
sperm,  and  such  changes  are  known  as  maturation.  The 
egg  cell  during  its  maturation  undergoes  mitotic  cell  divi- 
sion twice;  but  at  each  division  (&,  c),  instead  of  two  cells  of 
equal  size  being  formed,  one  cell  is  very  large  and  the  other 
minute.  The  smaller  cell  is  known  as  a  " polar  body," 
and  is  to  be  looked  upon  as  a  rudimentary  egg  which  has 
been  sacrificed  in  order  to  leave  the  functional  egg  as  much 
nourishment  as  possible.  The  maturation  of  the  sperm 
cell  (B)  consists  also  of  two  successive  cell  divisions  (b,  c), 
but  in  this  case  four  functional  spermatozoa  of  equal  size 
are  formed  (d),  each  bearing  a  little  tail  for  swimming. 

But,  you  ask,  why  not  have  the  egg  and  sperm  fuse  at 
once  without  ripening — why  have  maturation  at  all? 
There  is  apparently  but  one  important  thing  involved. 
During  the  two  cell  divisions  chromatin  material  is  elimi- 
nated and  the  number  of  chromosomes  is  reduced  to  one- 
half  the  somatic  number.  If  the  somatic  cells  in  a  par- 
ticular animal  showed  ten  chromosomes  during  mitosis, 
a  ripe  germ  cell  would  have  only  five. 


152 


@-p-g' 


GENERAL  ZOOLOGY 

X© 


H  /  J 

FIG.  64. — Scheme  for  the  developmental  stages  of  a  metazoan.  A,  B,  mat- 
uration of  egg  and  sperm  cells — a,  first  division;  6,  two  cells  resulting  from 
division;  c,  cells  after  next  division;  d,  ripe  egg  or  sperm  cell.  C,  zygote  formed 
by  union  of  ripe  egg  and  sperm,  now  dividing.  D,  cleavage  stages.  E,  section 
through  blastula  stage — the  stippled  cells  represent  the  portion  to  become  ento- 
derm.  F,  section  during  gastrula  formation;  x,  cleavage  cavity.  G,  section 
through  mature  gastrula;  ec.  ectoderm;  en,  entoderm;  y,  archenteron,  or  primitive 
digestive  cavity.  H,  young  metazoan  with  tubular  digestive  system.  H-K, 
stages  in  the  formation  of  the  coelom  or  body  cavity;  the  lower  figure  of  each 
stage  represents  a  cross  section  of  the  one  above;  co,  coelom. 


ONTOGENY  OF  METAZOA  153 

2.  Fertilization,  or  the  Formation  of  the  Zygote  (C). — 
After  maturation,   the  sperm  cells  move  to  the  eggs,  if 
conditions  permit,  and  fuse  with  them.     Only  one  sperm 
cell  unites  with  each  egg,  though  many  may  try.     The 
sperm  cell  is  always  much  smaller  than  the  egg.     Sometimes 
its  bulk  is  only  one  five-hundred  thousandth  that  of  'the 
ovum  with  which  it  unites.     It  usually  pierces  the  cyto- 
plasm of  the  egg  for  a  short  distance.     Before  fertilization 
takes  place  both  the  germ  cells  have  half  the  somatic 
number  of  chromosomes.     When  the  sperm  enters  the  egg, 
it  brings  chromatin  and  chromosomes  equal  to  those  in 
the  egg,  and  the  somatic  condition  is  thus  restored  in  the 
zygote. 

3.  Cleavage. — Soon  after  the  sperm  cell  has  entered  the 
egg   chromosomes   are   formed   in   both   egg   and   sperm 
nuclei,  a  spindle  is  originated  from  the  centrosomes  of  the 
sperm  nucleus,  and  the  chromosomes  take  position  around 
its  equator  (C).     These  chromosomes  are  half  from  the 
sperm  cell  and  half  from  the  egg.     The  usual  steps  in  mito- 
sis (page  40)  are  now  completed  arid  the  " fertilized  egg" 
or   zygote,   thus   divides  into  two  cells.     This  division  is 
known  as  the  first  cleavage.     There  follow  two-,  four-, 
eight-,  sixteen-,  thirty-two-celled  " cleavage  stages"  (D); 
and  others  involving  even  larger  numbers  of  cells.     After  a 
time  a  little  ball  of  cells,  the  morula,  is  formed  and  this 
later  becomes  hollow  by  having  a  water-filled  "  cleavage 
cavity"  (x)  form  in  the  middle.     Such  a  ball  of  cells  with 
a  central  cavity  is  called  a  blastula  (E).     It  soon  changes 
to  a  gastrula  by  the  turning  in  of  one  side. 

4.  Gastrulation;  the   Formation  of   the  Germ  Layers. — 
When  the  side  of  the  hollow  spheroidal  blastula  is  turned  in, 
the  cleavage  cavity  (x)  is  encroached  upon  and  is  finally 
completely   obliterated    because    the    turned    in   portion 
comes  in  contact  with  the  epithelium  on  the  other  side  of 
the  original  sphere  (F,  G).     The  result  is  a  concave  hemi- 
sphere made  up  of  two  layers.     The  epithelium  covering 
the  outside  is  the  ectoderm  (ec);  the  cavity  within  is  the 


154  GENERAL  ZOOLOGY 

archenteron  (?/),  or  primitive  digestive  cavity,  which  is 
lined  by  the  inner  epithelium  or  entoderm  (en).  The 
ectoderm  and  the  entoderm  are  the  primary  germ  layers, 
and  the  metazoan  is  now  a  gastrula  (G). 

Some  animals  (Fig.  68)  are  adults  in  the  gastrula  stage 
and  their  development  goes  no  farther.  They  remain  in  a 
diploblastic,  or  two-layered,  condition  all  their  lives  and 
the  archenteron  serves  their  simple  digestive  needs.  Most 
metazoans,  however,  have  a  third  layer,  the  mesoderm, 
formed  between  the  ectoderm  and  entoderm,  and  also  have 
modifications  of  the  archenteron  so  that,  instead  of  having 
only  one  opening  for  the  ingestion  and  rejection  of  food, 
a  mouth  and  anus  develop.  During  the  gastrula  stage, 
or  soon  after,  metazoans  take  on  the  distinctive  character- 
istics of  the  phylum  to  which  they  belong.  A  coelenterate 
remains  a  gastrula  but  forms  tentacles,  stinging  cells,  and 
other  structures  proper  for  its  adult  life.  Most  flatworms 
develop  a  great  mass  of  mesodermal  cells  but  retain  the 
original  opening  into  the  archeneteron  for  feeding.  More 
highly  specialized  animals  develop  far  beyond  the  gastrula 
stage;  the  three  germ  layers  becoming  greatly  modified 
by  thickenings,  the  formation  of  cavities,  foldings,  and  in 
other  ways. 

5.  The  Formation  of  the  Coelom,  or  Body-cavity. — After 
passing  through  the  gastrula  stage  a  coslomate  animal  de- 
velops a  tubular  digestive  system,  the  original  entrance 
into  the  archenteron  serving  as  one  opening  and  a  new 
aperture  forming  by  breaking  through  at  the  opposite 
end  (Fig.  64,  H).  Although  there  is  some  variation  in 
the  way  the  coelom  arises  in  different  groups  of  animals,  the 
following  description  will  serve  to  illustrate  the  general 
method.  Pairs  of  little  pockets  form  along  the  sides  of  the 
alimentary  canal  and  push  out  against  the  body  wall. 
The  cavities  in  these  later  lose  their  connection  with  the 
alimentary  canal  (taking  the  form  of  closed  sacs),  then 
enlarge  greatly  so  as  to  envelop  all  the  internal  organs 
(J),  and  fuse  together  (K).  Thus  a  large  space,  the  coelom 


ONTOGENY  OF  METAZOA  155 

(co)  is  formed  between  the  body  wall  and  that  of  the 
digestive  tube.  Organs  which  develop  later  as  outgrowths 
from  the  alimentary  canal  must  push  aside  the  membrane 
lining  the  coelom,  and  on  this  account  the  membrane  en- 
velops all  the  internal  organs  closely.  For  example,  the 
peritoneum,  or  lining  of  the  ccelom,  surrounds  all  the  ab- 
dominal organs  in  a  man's  body.  This  makes  it  difficult 
to  perform  surgical  operations,  for,  if  bacteria  gain  access 
to  the  ccelom,  they  will  cause  peritonitis  (inflammation  of 
the  peritoneum)  and  the  infection  will  wander  about  among 
the  internal  organs,  usually  causing  death. 

6.  The  Formation  of  Systems  of  Organs. — The  germ  layers 
play  rather  definite  roles  in  subsequent  development.  The 
nervous  tissues  and  the  outer  integument,  or  skin,  always 
arise  from  the  ectoderm.  The  entoderm  forms  the  lin- 
ing for  the  middle  portion  of  the  alimentary  canal,  or  midgut. 
Other  tissues  and  systems  of  organs  take  their  origin 
chiefly  from  the  mesoderm.  The  greater  part  of  the  body 
is  therefore  formed,  in  most  animals,  from  the  middle  germ 
layer. 

This  brief  synopsis  indicates  the  chief  steps  in  metazoan 
development :  Maturation,  fertilization,  cleavage,  gastrula- 
tion,  and  ccelom  formation.  The  first  three  stages  occur 
in  protozoans  as  well  as  metazoans,  but  it  is  only  in  many- 
celled  animals  that  tissue  differentiation  takes  place  after 
the  formation  of  the  primary  germ  layers  by  gastrulation. 
As  has  been  stated,  some  metazoans  never  get  much  beyond 
the  gastrula  stage,  but  even  those  that  pass  on  to  more 
specialized  conditions  (ccelom,  paired  appendages,  chorda, 
etc.)  follow  the  regular  route  through  the  stages  of  matura- 
tion, fertilization,  cleavage,  and  gastrulation.  This  per- 
sistent adherence  to  one  conventional  type  of  develop- 
ment is  believed  to  indicate  genetic  relationship  among 
animals.  According  to  the  Law  of  Biogenesis,  therefore, 
metazoans  arose  from  protozoans;  all  were  at  one  time  (and 
some  still  are)  gastrulas,  and  many  have  through  speciali- 
zation gone  farther.  The  racial  development  of  all  the 


156  GENERAL  ZOOLOGY 

phyla  of  animals  was,  therefore,  probably  the  same  up  to 
the  gastrula  stage  (with  one  exception,  and  this  will  be 
considered  in  the  next  chapter). 

Aside  from  the  development  of  the  ccelom,  the  most 
important  advances  in  the  recent  racial  history  of  animals 
are:  (1)  The  ascendency  of  the  bilateral  type  of  symmetry. 
(2)  The  development  of  paired  appendages.  (3)  The  more 
"progressive"  bilaterally  symmetrical  animals  are  all 
metameric,  and  the  simpler  metameric  types  consist  of  a 
similar  series  of  segments  from  the  anterior  to  the  posterior 
end.  This  is  homonomous  metamerism,  such  as  is  present 
in  the  earthworm.  (4)  In  the  most  specialized  phyla 
of  metameric  animals,  however,  there  is  more  importance 
given  to  the  segments  toward  the  anterior  end  (i.e.j  there  is 
progressive  cephalization),  and  this  often  results  in  the 
formation  of  a  distinct  head  which  bears  important  sense 
organs.  Appendages  develop  first,  both  phylogenetically 
and  ontogenetically,*  as  mere  outgrowths  or  flaps  from  the 
body  and  may  later  become  jointed  or  modified  in  various 
ways. 

*  Phylogeny  is  racial  development;  ontogeny  is  individual  development. 


CHAPTER  XIV 
PHYLUM  PORIFERA— SPONGES 

Porifera  means  "  pore-bearers, "  and  the  sponges  could 
not  have  been  better  named.  The  body  of  any  sort  of  a 
sponge  is  full  of  little  canals  which  open  through  minute 
apertures  over  the  whole  outside  surface  (Fig.  65).  On 
the  interior  the  canals  connect  with  larger  channels  which 


A  B 

FIG.  65. — Structure  of  a  simple  sponge.  A,  half  of  an  animal  split  length- 
wise to  show  the  central  longitudinal  canal  and  the  lateral  canals  (p)  leading  into 
it.  B,  enlarged  section  of  a  sponge's  body  wall  showing  different  types  of  cells; 
a,  amoebocyte;  6,  undifferentiated  cell;  c,  collared  flagellate  cell,  or  choanocyte; 
e,  epidermis  on  outside  of  body;  s,  spicule  within  the  cell  that  formed  it. 

lead  again  to  the  outside.  Certain  portions  of  the  internal 
canals  are  lined  with  peculiar  cells,  the  choanocytes,  which 
each  bear  a  flagellum  and  a  flexible  collar  surrounding  it 
(B,  c).  The  flagella  on  all  of  these  cells  wave  constantly 
and  create  currents  in  the  canals.  Most  sponges  have  a 

157 


158  GENERAL  ZOOLOGY 

skeleton,  which  may  be  composed  of  separate  spicules  of 
lime  or  silica,  or  consist  of  a  horny  network.  In  their  man- 
ner of  growth  sponges  are  much  like  plants,  living  attached 
to  some  object  and  varying  their  branches  to  suit  the 
surroundings.  The  natural  history  of  a  simple  representa- 
tive of  the  Porifera  will  give  some  idea  of  the  general  ac- 
tivities of  these  peculiar  metazoans. 

A  SIMPLE  SPONGE 

Self -maintenance. — A  sponge  obtains  food  from  the 
water  continually  passing  through  its  canals  (Fig  65). 
Little  plants  and  animals  are  sucked  into  the  pores  from  the 
surrounding  water,  are  drawn  inside  the  collars  of  the  choa- 
nocytes  or  engulfed  by  other  cells  as  they  pass  through  the 
canals.  There  is  no  digestive  system,  or  digestive  cavity, 
no  mouth  or  anus.  A  cell  lining  a  canal  takes  food  into 
its  cytoplasm  much  as  an  amoeba  does;  digestion  takes 
place  only  within  the  cells.  The  choanocytes  and  some 
other  cells  lining  the  canals  are,  therefore,  the  only  ones 
able  to  feed,  and  they  must  pass  on  nourishment  to  the 
rest  of  the  sponge  body.  There  is  no  circulatory  system  to 
distribute  food;  transfer  takes  place  through  accidental 
spaces  between  cells  and  by  gradually  soaking  from  one 
cell  into  the  next.  Excretion  occurs  through  exposed  sur- 
faces, and  is  greatly  assisted  by  wandering  amoeboid  cells 
which  engulf  waste  particles  and  migrate  outside  the  sponge's 
body  with  them. 

Self -protection. — Sponges  are  not  very  palatable  morsels 
for  other  animals,  and  hence  have  few  enemies  that  seek  to 
devour  them.  They  are  usually  protected  by  the  sharp 
spicules  in  all  parts  of  their  bodies.  The  spicules  are  ac- 
cretions formed  by  cells  which  die  after  the  work  is  completed 
(Fig.  65,  B,  c).  They  not  only  protect  the  body,  but  also 
help  to  support  it. 

Sponges  grow  attached  and  hence  cannot  migrate  to  new 
habitats  if  their  surroundings  become  unfavorable.  They 


PHYLUM  PORIFERA— SPONGES  159 

must  be  able  to  resist  injurious  conditions,  or  die.  A  large 
part  of  the  body  may  disintegrate  when  the  water  is  foul  or 
when  food  fails;  but  if  a  few  cells  are  left,  they  will  grow  into 
a  complete  new  sponge,  just  as  a  tree  may  send  up  a  shoot 
from  its  stump  after  being  cut  down.  Sponges,  then, 
usually  have  little  power  of  resistance  but  possess  remark- 
able ability  to  regenerate  lost  parts.  Professor  H.  V. 
Wilson  has  recently  succeeded  in  growing  complete  sponges 
from  minute  pieces  which  had  been  strained  through  a  fine 
sieve.  The  bodies  of  fresh-water  sponges  disintegrate  in  the 
autumn,  but  certain  groups  of  cells,  the  gemmules,  form 
resistant  coats  about  themselves,  remain  dormant  until 
spring,  and  then  grow  into  new  sponges. 

Race  Preservation. — As  has  just  been  stated,  sponges 
have  remarkable  powers  of  regeneration.  Many  species  are 
even  able  to  throw  off  parts  of  the  body  which  grow  into 
complete  individuals.  In  a  large  sponge  it  is  often  impos- 
sible to  determine  how  many  individuals  are  present  for  the 
canals  are  interwoven  and  cross-connected  inextricably. 
Sponges  may  be  easily  separated  into  parts  without  serious 
injury  and  different  individuals  are  as  readily  fused  together. 
All  the  methods  of  reproduction  thus  far  described  are 
strictly  asexual — there  is  no  fusion  of  sexual  cells  to  form  a 
zygote,  no  maturation,  fertilization,  or  cleavage.  Sexual 
reproduction  does  occur  in  sponges,  however,  and  most 
individuals  start  in  the  beginning  from  a  fertilized  egg. 

The  ova  and  spermatozoa  of  sponges  ripen  during  typical 
maturation  stages,  as  described  in  the  last  chapter,  and 
fertilization  takes  place  as  in  other  metazoans.  Though 
the  cleavage  stages  are  somewhat  peculiar,  they  are  much 
like  those  in  other  many-celled  animals.  At-  this  point, 
however,  the  resemblance  between  sponge  development  and 
that  of  other  Metazoa  ceases,  for  no  gastrula  is  formed.  In 
other  words  all  metazoans,  except  sponges,  pass  through 
maturation,  fertilization,  cleavage,  and  gastrula  stages;  the 
sponges  pass  through  the  first  three,  but  form  no  gastrula. 
This  is  taken  to  mean  that  all  many-celled  animals  except 


160  GENERAL  ZOOLOGY 

sponges  are  related,  and  that  the  Porifera  arose  independ- 
ently from  the  Protozoa.  The  Metazoa  are,  therefore, 
divided  into  two  groups  (page  24) :  the  Enterozoa,  in- 
cluding most  of  the  phyla;  and  the  Parazoa,  containing  only 
the  sponges. 

These  theoretical  matters  regarding  classification  are  not 
to  the  point,  however,  and  we  will  return  to  the  develop- 
ment of  the  sponge.  A  ciliated  larva  is  formed  within  a 
special  cavity,  or  brood  pouch,  after  cleavage  inside  the 
body  of  its  parent.  The  larva  when  set  free  leaves  its 
parent's  body  in  the  water  currents  and  swims  about 
actively  until  it  finds  some  suitable  place  for  attachment; 
then  settles  down,  loses  its  ciliated  covering,  develops  pores 
through  the  tissues,  and  grows  into  a  mature  sponge. 

In  its  mature  state  a  sponge  is  barely  across  the  line  be- 
tween the  Protozoa  and  the  Metazoa.  It  has  tissues,  but 
that  is  about  all  that  can  be  granted  it !  There  is  little  or 
no  correlation  between  the  different  parts  of  the  body,  and 
there  are  no  organs.  Each  tissue  does  its  work  more  or  less 
automatically  and  is  rather  independent  of  others.  The 
flagellated  choanocytes  beat  continually  until  they  die,  and 
will  always  make  currents  if  the  canals  are  open.  If  some 
injurious  substance  is  entering  with  the  water,  it  can  be 
excluded  only  by  closing  the  openings,  and  such  closure  is 
brought  about  by  the  direct  action  of  the  substance  on  the 
little  sphincters,  or  rings  of  muscular  tissue,  surrounding 
the  openings.  If  a  certain  part  of  a  sponge's  body  is  cut,  it 
may  contract  and  adjacent  pore  openings  may  close,  but 
there  is  no  transmission  of  the  disturbance  to  other  parts  of 
the  body.  The  sponge  is,  therefore,  a  rather  poorly  organ- 
ized community  of  tissues.  Muscular  and  supporting 
tissues  are  present  but  there  are  no  nerves,  nor  are  there 
evidences  of  nervous  transmission.  This  .is  an  interesting 
fact  from  a  psychological  point  of  view  for  it  indicates  that 
the  nervous  system  in  animals  generally  has  arisen  after 
various  effectors  (muscles,  glands,  etc.),  and  has  secondarily 
come  to  control  them.  A  sponge  has  effectors  (parts  of  the 


PHYLUM  PORIFERA— SPONGES  161 

body  capable  of  doing  work),  but  has  no  nervous  mechanism 
to  control  them  and  they  respond  when  stimulated  directly. 
In  more  specialized  animals  the  nervous  system  has  more  or 
less  complete  control  of  effectors — the  receptors  (eye,  ear, 
etc.)  receive  stimuli  and  send  impulses  to  the  central  nervous 
system,  which  in  turn  sets  off  appropriate  effectors. 

SPONGE  FISHERY 

The  total  value  of  the  sponge  fisheries  of  the  world 
amounts  to  about  $3,500,000  annually.  In  the  year  1907 
sponges  were  imported  to  the  United  States  to  the  value  of 
$488,426  and  exports  during  the  same  year  amounted  to 
$144,354.  In  1908,  on  the  Florida  fishing  grounds  alone, 
622,489  pounds  of  sponges  were  captured  and  sold  for 
$548,876.  The  growing  importance  of  the  sponge  fisheries 
has  led  the  federal  government  to  make  studies  of  the  best 
methods  of  propagation  and  fishing. 

It  is  possible  to  raise  sponges  from  slips,  like  plants,  and 
this  has  been  made  use  of  in  a  commercial  way.  Selected 
individuals  of  good  quality  are  cut  up  and  the  pieces  placed 
on  the  tips  of  sharp  stakes  attached  to  a  frame.  The  frame 
is  lowered  to  the  bottom  of  the  ocean,  left  for  a  year  or  two, 
and  raised  again.  If  all  has  gone  well,  the  "  slips "  have 
grown  to  marketable  size  and  are  of  good  shape  (Fig.  66). 
It  is  possible  to  graft  fragments  of  fine  sponges  on  the  at- 
tached growing  bases  of  Bother  indviduals  of  poor  quality. 
The  two  sponges  fuse  and  a  vigorous  growth  of  the  desirable 
portipn  is  thus  obtained. 

Sponges  are  usually  collected  by  hooking  or  diving.  In 
the  first  method  one  man  rows  a  boat  and  another  uses  a 
rake-like  hook  to  tear  the  sponges  from  their  attachments. 
In  the  second,  divers  go  down  and  cut  the  sponges  off  the 
bottom  by  hand.  Along  the  shores  of  the  United  States 
fishers  usually,  wear  diving  suits,  but  in  the  Mediterranean 
and  other  seas  they  are  more  often  nude.  In  some  places 
dredges  are  used  in  capturing  sponges,  but  they  are  un- 


162 


GENERAL  ZOOLOGY 


desirable  because  they  destroy  great  numbers  of  young 
individuals.  Sponges  soon  die  on  the  deck  of  a  vessel,  and 
are  allowed  to  rot  for  a  day  or  two.  They  are  then  hung 
over  the  side  of  the  ship  or  placed  in  "  crawls, "  or  cages,  in 
the  water  alongshore  to  macerate  until  the  flesh  is  very  soft. 


FIG.  66. — Showing  sponges  grown  from  "slips"  on  spikes  set  in  cement  blocks 
and  left  for  a  time  on  the  bottom  of  the  ocean.      (From  Moore.) 

When  thoroughly  rotted,  the  sponges  are  beaten  and 
squeezed  under  water.  They  are  then  strung  in  bunches 
and  auctioned  off  to  buyers  on  the  wharves.  In  the  packing 
houses  the  sponges  are  trimmed,  cleaned,  sorted  according 
to  size  and  quality,  and  sometimes  bleached.  They  are 
then  ready  for  the  consumers. 


CHAPTER  XV 

PHYLUM  CCELENTERATA;  PHYLUM  CTE- 
NOPHORA 

The  coelenterates  are  all  modified  gastrulas  which  are 
radially  symmetrical,  and  possess  peculiar  "  stinging  cells," 
or  nematocysts.  They  are  all  aquatic,  and  most  of  them 
are  marine,  only  three  species  occurring  in  fresh  water. 
Well-known  examples  of  the  phylum  are  polyps,  such  as 
corals  and  hydras,  and  the  jellyfishes,  or  medusae.  In  many 
species  there  is  an  alternation  of  generations — a  polyp 
stage  giving  rise  to  a  medusa,  and  it  in  turn  to  a  polyp 
again.  There  are  three  classes:  Hydrozoa,  Scyphozoa, 
and  Anthozoa. 

CLASS  1.     HYDROZOA 

On  account  of  the  bush-like  forms  often  assumed  by  the 
colonies  of  polyps  when  growing  alongshore,  the  coelen- 
terates in  this  class  were  long  known  as  " zoophytes" 
or  ' '  animal  plants."  A ' '  bush  "  may  bear  hundreds  or  even 
thousands  of  little  polyps  at  the  tips  of  the  branches,  and 
all  are  connected  through  the  twigs  with  the  main  stalk  of 
the  colony  which  is  attached  to  some  solid  object.  Though 
polyps  are  the  most  prevalent  types  of  animals  in  this 
class,  minute  medusae  are  usually  produced  at  intervals. 
All  medusae  are  shaped  more  or  less  like  an  umbrella  and 
those  of  the  Hydrozoa  are  easily  distinguished  from  others 
by  the  presence  of  a  velum — a  shelf-like  structure  pro- 
jecting inward  around  the  entire  margin  of  the  umbrella 
(Fig.  68,  v).  Along  the  Atlantic  coast  of  the  United  States 
a  little  hydrozoan  grows  very  commonly  on  docks,  piles, 
and  seaweeds.  This  is  Obelia  geniculata  (Linnaeus),  a 
Colonial  Hydroid,  which  may  well  serve  as  an  example  of 

163 


164 


GENERAL  ZOOLOGY 


this  class.  This  hydroid  always  grows  as  a  branching 
colony  (Fig.  67).  All  individuals  are  intimately  associ- 
ated with  each  other  and  the  food  taken  by  one  animal 
may  pass  down  the  hollow  stalk  to  nourish  other  members 
of  the  colony.  Such  cooperative  activity  between  different 


FIG.  67. — Obelia,  a  colonial  hydrozoan.  A,  a  small  colony  somewhat  enlarged; 
B,  portion  of  same  greatly  enlarged;  6,  medusa  buds;  c,  coenosarc,  or  soft  flesh; 
e,  old  branch  from  which  an  individual  has  broken  off;  g,  gonangium,  or  repro- 
ductive individual ;  h,  a  mature  hydranth ;  or  feeding  individual ;  j,  joints  to  permit 
bending  of  hydranth  on  its  stem;  ra,  young  medusa  swimming  out  of  gonangium; 
p,  perisarc;  y,  young  hydranth. 

individuals  makes  division  of  labor  possible  and  in  Obelia 
differentiation  of  functions  has  gone  so  far  that  some  mem- 
bers of  the  colony  carry  on  reproduction  and  cannot  feed, 
others  only  feed  and  cannot  reproduce.  There  are,  then, 


PHYLUM  CCELENTERATA 


165 


two  kinds  of  individuals  at  the  ends  of  the  branches: 
feeding  hydranths  (h)'  and  gonangia  (g) .  Obelia  is  therefore 
an  example  of  polymorphism,  a  condition  in  which  a  single 
species  appears  in  more  than  one  form. 


Po/yp 


Medusa 


FIG.  OS. — Indicating  the  fundamental  structural  similarity  between  gastrula, 
polyp,  and  medusa.  The  polyp  is  a  gastrula  with  tentacles;  the  medusa  is  like 
a  polyp  that  has  grown  upward  around  the  tentacles  in  the  region  of  x.  a, 
archenteron,  or  primitive  digestive  cavity;  ra,  mouth. 

It  has  been  said  that  all  ccelenterates  are  modified  gas- 
trulas,  and  this  will  hold  true  for  both  hydranths  and 
gonangia.  In  a  hydranth  a  circlet  of  tentacles  has  grown 
about  the  gastrula 's  mouth,  which  is  simply  the  original 
opening  into  the  archenteron  (Fig.  64,  G).  The  gonangium 


166 


GENERAL  ZOOLOGY 


has  developed  no  tentacles,  but  has  little  knobby  buds 
along  its  sides.  Both  sorts  of  individuals  have  only  two 
layers  (ectoderm  and  entoderm)  in  the  body  wall.  The 
hydranth  is  a  polyp  (Fig.  68)  which  differs  but  little  from  a 
gastrula,  except  for  the  connections  it  has  with  the  other 
individuals  in  a  colony. 

Self -maintenance. — The  Obelia  hydranths  are  the  only 
individuals  in  a  colony  capable  of  feeding.  Each  one 
catches  little  animals  in  its  tentacles  and  pokes  them 
through  its  mouth  into  the  simple  digestive  cavity.  There 


FIG.  69. — A  cnidoblast,  or  cell  containing  a  nematocyst.     A,  with  nematocyst 
coiled;  B,  nematocyst  discharged;  c,  cnidocil,  or  trigger. 


is  no  opportunity  to  move  about  to  look  for  food,  hence 
Obelia  always  grows  where  there  are  currents  and  waves,  so 
that  food  may  be  brought  to  it.  If  the  animals  captured  are 
too  lively  and  struggle  about,  they  are  stung  by  the  nema- 
tocysts  which  cover  the  tentacles.  The  nematocysts 
of  ccelenterates  (Fig.  69)  contain  a  poison  which  soon 
kills  small  animals,  so  that  they  can  be  swallowed  without 
trouble. 

After  the  food  enters  the  enteron,  or  digestive  cavity 
(Fig.  68,  a)  it  is  kept  circulating  by  the  movement  of  little 
flagella  on  certain  cells  in  the  epithelial  lining  (=  ento- 


PHYLUM  CCELENTERATA  167 

derm).  Other  cells  pour  out  secretions  which  soften  and 
liquefy  the  food.  If  there  are  parts  which  cannot  be 
digested  they  must  be  thrown  out  through  the  mouth  for 
there  is  no  other  opening  into  the  enteron,  except  the  canal 
which  leads  down  the  stalk  to  connect  with  other  indi- 
viduals. The  digested  food  is  absorbed  by  the  entoderm 
and  part  of  it  is  there  assimilated,  but  some  soaks  on 
through  to  nourish  the  ectoderm  on  the  outside  of  the 
body.  The  layers  of  Obelia's  body  wall  are  so  thin  that 
there  is  no  need  for  any  separate  circulatory  or  excretory 
systems. 

Self-protection.— An  Obelia  colony  is  invested  over  its 
entire  surface  by  the  peritheca,  a  tough  horny  secretion 
which  forms  a  branched  system  of  tubes  and  protects  the 
soft  tissues  within.  This  exoskeleton  has  little  openings 
at  the  tips  of  the  branches  so  that  the  hydranths  and 
gonangia  may  communicate  with  the  exterior.  Each  hy- 
dranth  is  enclosed  by  a  little  cup,  or  hydrotheca  (Fig.  67,  ti) 
into  which  it  may  withdraw  when  threatened  with  danger. 
The  peritheca  also  keeps  the  delicate  body  from  drying  out, 
if  it  is  by  chance  left  exposed  at  low  tide.  If.  this  protective 
covering  were  too  rigid,  it  would  be  a  burden  and  is  there- 
fore jointed  (j)  near  the  base  of  each  branch  so  that 
movement  is  permitted. 

The  nematocysts  (Fig.  69)  help  to  protect  Obelia  from 
small  creatures  which  might  otherwise  prey  upon  it,. but 
some  animals  appear  to  be  immune  to  their  feeble  stings. 
Certain  crabs  use  Obelia  and  other  hydroids  as  a  favorite 
food.  Some  snails  which  have  no  shells  not  only  devour  the 
hydroids,  but  do  so  without  discharging  the  nematocysts; 
and  some  species  are  even  able  to  pass  them  out  to  their 
own  backs  where  they  use  them  "  second-hand "  to  sting 
their  enemies. 

The  attachment  of  Obelia  to  some  solid  object  protects 
it  from  certain  dangers.  It  cannot  be  swept  up  on  the 
beach  by  storms  and  is  never  carried  out  to  sea,  but  it 
thus  sacrifices  its  ability  to  search  about  for  food. 


168  GENERAL  ZOOLOGY 

Race  Preservation. — Obelia  has  a  rather  complicated  life 
cycle  because  there  is  always  an  alternation  of  polyp  and 
medusa.  The  polyp  generation  reproduces  by  asexual 
methods.  It  starts  as  one  individual  which  produces  a  great 
number  of  others  by  budding ;  some  remain  attached  to  the 
colony  and  others  are  set  free.  The  medusa  produces 
zygotes  which  grow  into  young  and  vigorous  polyps.  Such 
an  alternation  of  a  generation  which  reproduces  asexually 
by  budding  with  another  generation  which  produces  new 
individuals  by  sexual  methods  is  known  as  metagenesis. 

Metagenesis  in  Obelia  takes  the  following  sequence  (Fig. 
71):  The  zygote  (A)  develops  in  the  way  described  for  a 
typical  metazoan,  passing  through  blastula  and  gastrula 
stages,  and  finally  forms  a  ciliated  larva  (B)  which  swims 
about  for  a  time,  then  attaches  itself  to  some  object  along 
the  shore.  The  larva  is  transformed  into  a  little  polyp 
(C),  which  soon  starts  to  feed  and  forms  a  peritheca  about 
itself.  As  it  grows  the  polyp  buds  off  hydranths  and 
gonangia  until  a  great  branching  colony  is  formed  (D). 

When  the  gonangia  on  a  colony  are  mature,  they  are 
continually  forming  medusa  buds  (Fig.  67,  &).  These 
ripen  into  little  medusse  (Fig.  71,  E)  which  break  loose  and 
swim  out  through  the  opening  at  the  end  of  the  gonangium. 
A  medusa  swims  by  pulsating  its  umbrella,  and  is  able  to 
feed  on  small  animals.  It  swims  feebly,  however,  and  is 
often  swept  a  long  way  by  ocean  currents.  Though  some 
individuals  may  die  before  the  ends  of  their  journeys,  such 
traveling  is  of  benefit  in  general,  for  Obelia  is  thus  able  to 
become  established  in  new  localities.  Each  medusa  is 
either  a  male  or  female.  The  females  give  off  zygotes 
formed  by  the  union  of  egg  and  sperm  cells  and  the  cycle 
thus  begins  again. 

In  metagenesis,  then,  as  illustrated  by  Obelia,  there  is 
always  an  alternation  of  an  asexual  polyp  generation  with 
a  sexual  medusa  generation.  Polyps  cannot  form  other 
polyps  directly  except  by  budding  and  medusae  must 
always  produce  germ  cells  which  grow  into  polyps.  At  first 


PHYLUM  CCELENTERATA  169 

glance  one  generation  is  quite  different  in  structure  from 
the  other,  but  if  some  details  are  ignored,  striking  simi- 
larities between  polyp  and  medusa  are  apparent.  Fig.  68 
shows  that  a  simple  polyp  may  readily  be  transformed  to 
a  medusa  by  two  changes — an  additional  ring  of  tentacles 
must  grow  out  at  xt  and  the  part  of  the  polyp  bearing  the 
mouth  and  tentacles  will  have  to  be  pushed  back  into  a  cup 
formed  by  an  outgrowth  at  the  bases  of  the  new  set  of 
tentacles. 

If  we  compare  Obelia  with  the  sponge  described  in  the  last 
chapter,  it  is  at  once  apparent  that  we  are  dealing  with 
a  more  progressive  animal.  The  sponge  excels  Obelia 
only  in  its  great  elaboration  of  tissues.  Though  a  hydroid 
has  its  tissues  rather  poorly  defined,  it  shows  progress  in 
other  ways:  there  is  a  definite  digestive  cavity;  the  ten- 
tacles are  organs,  for  they  consist  of  different  tissues  which 
cooperate  to  capture  food  and  poke  it  into  the  enteron. 
There  is  coordination  between  different  parts  of  the  body 
in  a  hydroid,  whereas  a  sponge  must  have  each  of  its  tis- 
sues activated  by  a  direct  stimulus.  If  meat  juice  is 
squirted  in  the  water  near  an  Obelia,  the  tentacles  wave 
toward  the  source  of  the  extract  and  the  mouth  is  also 
bent  in  the  same  direction.  If  the  tip  of  a  tentacle  is  in- 
jured the  whole  hydranth  withdraws  within  the  protection 
of  the  peritheca.  A  sponge  is  a  rather  loosely  coordinated 
federation  of  tissues;  a  hydroid  is  a  well-organized  metazoan . 
with  a  definite  digestive  cavity  and  simple  organs. 

Obelia  may  be  looked  upon  as  an  average  hydroid.  Other 
representatives  vary  greatly  in  the  degree  to  which  poly- 
morphism and  metagenesis  have  been  developed.  The 
fresh-water  Hydra  (Fig.  68,  Polyp)  shows  a  very  simple 
condition.  It  consists  of  a  simple  polyp  which  may  at  times 
form  new  individuals  by  budding  and  at  other  times  give 
off  eggs  or  spermatozoa.  The  zygotes  grow  into  polyps  and 
there  is  thus  no  metagenesis  and  no  polymorphism.  The 
Portuguese  man-of-war,  Physalia  (Fig.  70),  which  floats 
in  tropical  seas,  shows  an  extreme  case  of  polymorphism. 


170  GENERAL  ZOOLOGY 

In  this  hydroid  there  is  a  bladder  which  bears  a  colony 
of  hydroids  beneath  and  there  are  five  kinds  of  individuals. 
The  nematocysts  of  Physalia  are  very  poisonous;  bathers 
swimming  against  the  trailing  submarine  batteries  experi- 
ence great  pain. 


FIG.  70. — The  Portuguese  man-o'-war.  A,  general  view;  B,  sectional  diagram 
to  show  different  kinds  of  individuals  in  the  colony;  e,  gasterozoid,  or  feeding 
individual;  /,  dactylozoid;  g,  dactylozoid  with  tentacle;  i,  female  reproductive 
individual;  n,  male  reproductive  individual. 


CLASS   2.     SCYPHOZOA 

In  scyphozoans  the  medusa  stage  is  always  much  more 
important  than  the  polyp.  The  jellyfishes  do  not  have 
a  velum,  and  many  of  them  are  very  large;  Cyanea  capil- 
lata,  which  occurs  off  the  New  England  coast,  attaining  a 
diameter  of  six  feet.  The  polyp  of  a  scyphozoan  is  al- 
ways small  and  inconspicuous.  It  forms  medusae  by  a 
peculiar  type  of  budding  known  as  strobilization,  i,e., 
constrictions  are  formed  about  the  body  so  that  it  looks 
like  a  pile  of  saucers;  the  top  " saucer"  periodically  breaks 
off  and  swims  away  as  a  young  medusa  (Fig.  71,  D). 

On  account  of  their  large  size  and  great  abundance  in 
many  parts  of  the  ocean,  scyphozoans  have  been  much  used 
for  experimental  work  in  zoology  and  physiology.  One 


PHYLUM  CGELENTERATA 


171 


group  of  experiments  dealt  with  the  control  of  the  rhythmical 
pulsation  of  the  umbrella.  There  are  eight  nerve  centers 
around  the  margin  of  a  jellyfish;  these  control  the  pulsa- 


B 


HYDROZOA  SCYPHOZOA  ANTHOZOA 

FIG.  71. — Life  histories  of  typical  coelenterates.  A,  zygote;  B,  ciliated 
planula;  C,  young  polyp;  D,  mature  polyp;  E,  medusa.  In  the  Hydrozoa  all 
stages  usually  appear  in  a  well  developed  condition;  in  the  Scyphozoa  the  polyp 
is  always  small  and  unimportant;  the  Anthozoa  never  have  a  medusa  stage. 

tion  and  all  are  correlated  so  that  the  area  dominated  by 
each  beats  synchronously  with  other  regions.     If  all  but 


172  GENERAL  ZOOLOGY 

one  of  the  centers  are  removed  the  whole  umbrella  keeps 
on  beating  as  before ;  if  all  the  centers  are  removed,  however, 
there  is  no  more  pulsation.  The  medusa  therefore  has  a 
very  wide  margin  of  safety  in  case  of  injury. 

CLASS  3.     ANTHOZOA 

The  Anthozoa  have  no  medusa  stage,  there  is,  in  fact,  no 
indication  of  metagenesis.  The  polyps  produce  eggs  and 
spermatozoa  which  form  zygotes,  and  these  grow  into 
polyps  like  their  parents  (Fig.  71,  D).  Anthozoan  polyps 
are  more  complicated  than  those  in  any  other  group. 
There  is  a  gullet  leading  from  the  mouth  into  the  digestive 
cavity,  which  is  divided  up  into  small  compartments  by 
thin  partitions. 

The  anemones  are  solitary  polyps  which  form  no  skeleton 
whereas  corals  are  colonial  anthozoans.  Corals  live  in 
large  colonies,  often  showing  great  polymorphism,  and 
secrete  a  skeleton  of  lime  or  other  substances.  Many 
islands  and  atolls  have  been  formed  entirely  from  the  skele- 
tons of  corals  during  periods  when  the  bottom  of  the  ocean 
was  slowly  sinking. 

GENERAL  REMARKS  ON  OELENTERATES 

When  considered  in  relation  to  the  Protozoa  and  Porifera, 
the  phylum  Ccelenterata  is  remarkable  for  several  reasons. 
The  following  points  should  be  particularly  kept  in  mind: 
(1)  though  many  Ccelenterata  are  very  complicated  in  struc- 
ture, none  is  much  more  than  a  modified  gastrula;  (2) 
though  tissues  are  rather  poorly  developed  in  some,  others 
have  well-defined  supporting,  nervous  and  muscular  tissues; 
(3)  organs  are  present;  (4)  a  definite  cavity,  the  enteron,  is 
used  for  digestion  and  it  presents  the  simplest  condition  to 
be  found  in  any  of  the  Enteroccela  (Fig.  18,  page  24),  having 
but  one  opening;  (5)  colony  formation  and  polymorphism 
are  common ;  (6)  metagenesis,  or  alternation  of  a  sexual  and 


PHYLUM  CTENOPHORA  173 

an  asexual  generation,  is  here  developed  as  it  is  nowhere  else 
in  the  animal  kingdom. 

PHYLUM  CTENOPHORA 

The  ctenophores,  comb-jellies,  or  sea- walnuts  (Fig.  72), 
are  extremely  transparent,  jelly-like  animals  which  swim 
at  the  surface  of  the  pcean.  They  are  often  shaped  like  a 
walnut,  as  one  of  their  names  indicates,  and  have  two  re- 
tractile tentacles  which  trail  behind  as  they  swim.  There 


FIG.  72. — A  ctenophore. 

are  eight  rows  of  comb-like  paddle  plates  which  propel  the 
body  slowly  through  the  water.  Ctenophores  show  a 
slight  superiority  over  coelenterates  in  the  structure  of  the 
digestive  system  by  having  an  opening,  the  anus,  at  the 
opposite  end  from  the  mouth.  They  are  usually  protected 
from  enemies  by  their  extreme  transparency,  which  renders 
them  practically  invisible  during  the  day.  Many  species 
are  phosphorescent,  and  give  off  light  when  disturbed. 


CHAPTER  XVI 


PHYLUM  PLATYHELMIA 

Flatworms  are  bilaterally  symmetrical  and  usually  have 
a  broad  leaf-like  form.  Some  have  a  digestive  cavity  into 
which  there  is  only  one  opening,  the  mouth ;  in  tapeworms, 
however,  no  digestive  system  is  present.  There  are  three 
classes: 

Class  1.  Turbellaria;  free  living  Platyhelmia  with  cilia  on  the  out- 
side of  the  body  and  a  well-developed  digestive  system. 

Class  2.  Trematoda,  or  flukes;  parasitic  flatworms,  with  a  tough 
cuticle  on  the  outside  of  the  body  and  no  cilia. 

Class  3.  Cestoidea,  or  tapeworms,  which  are  parasitic,  have  a  cuticu- 
lar  covering,  and  lack  any  digestive  system. 

CLASS  I.    TURBELLARIA 

Turbellarians  are  the  only  flatworms  that  are  not  para- 
sitic. They  live  for  the  most  part  in  quiet  places  among 
vegetation,  both  in  the  ocean  and  in  fresh- water,  and  seek 
actively  for  small  animals  which  are  swallowed  whole. 
The  fresh-water  planarians  are  probably  the  most  familiar 
representatives,  and  the  activities  of  one  of  these  worms  will 
be  considered. 

Planaria  maculata  Leidy 

Self -maintenance. — This  turbellarian  (Fig.  73)  frequents 
weedy  ponds,  springs,  and  the  shores  of  lakes.  It  crawls 
'  about  among  aquatic  plants  looking  for  food.  Locomotion 
is  accomplished  by  the  movement  of  cilia  covering  the  out- 
side of  the  body  and  is  also  greatly  facilitated  by  the  slime, 
which  gives  the  worm  a  firm  enough  hold  on  solid  objects  so 

174 


PHYLUM  PLATYHELMIA 


175 


that  the  body  may  be  extended  and  contracted  by  muscular 
movements.  A  planarian  crawls  about  until  it  encounters 
some  little  animal  suitable  for  food.  The  proboscis  is  ex- 
tended through  the  mouth  (Fig.  73),  wraps  about  the  food, 
and  is  then  drawn  into  the  body.  Though  a  planarian  has 
a  pair  of  simple  eyes,  they  are  not  used  to  any  extent  in 
seeking  or  capturing  food,  but  it  does  have  the  ability  to 
recognize  food  through  chemical  substances  given  off  in 
the  water.  If  an  injured  animal  is  encountered  which  is 
bleeding  slightly,  it  will  be  followed  about  persistently,  but 


FIG.  73. — Planarians.  The  individual  at  the  left  id  in  the  act  of  swallowing 
a  small  crustacean.  On  the  right  is  a  planarian  which  has  had  its  head  cut  off; 
the  body  has  regenerated  a  new  head,  but  the  small  fragment  has  formed  a  two- 
headed  monster. 

a  sound  and  whole  individual  may  not  be  noticed.  Pla- 
naria's  mouth  is  in  a  rather  strange  position,  at  the  center 
of  the  ventral  surface,  but  that  situation  gives  good  oppor- 
tunity to  stretch  the  proboscis  out  in  any  direction. 

After  food  has  been  drawn  into  the  body  it  passes  down 
the  proboscis,  or  pharynx,  into  the  digestive  cavity  which 
has  three  chief  branches — one  leading  forward  to  the  ante- 
rior end  of  the  body  and  two  leading  back  on  either  side 
of  the  mouth  to  the  opposite  end.  The  three  main  canals 
have  many  small  side  pouches,  and  food  is  thus  distributed 
in  the  digestive  system  to  all  parts  of  the  body.  It  is, 


176  GENERAL  ZOOLOGY 

therefore,  unnecessary  to  have  a  separate  circulatory  system, 
and  none  is  present.  Planaria  does  have  a  very  elaborate 
excretory  system,  however,  which  consists  of  a  branching 
tube  on  either  side  of  the  body.  The  branches  end  in 
"flame  cells;"  so  named  because  each  is  hollow  and  has  a 
bunch  of  cilia  within  which  resemble  the  flickering  of  a 
candle,  when  moving.  The  flame  cells  force  waste  prod- 
ucts down  the  tubes  and  ultimately  out  of  the  body  through 
two  little  pores  on  the  dorsal  surface. 

Self -protection. — Planaria  escapes  most  of  its  enemies  by 
hiding.  Its  eyes  are  of  value  in  enabling  it  to  shun  light 
and  seek  dark  crevices,  but  the  whole  surface  of  the  body 
is  also  sensitive  and  a  planarian  can  respond  nearly  as  well 
after  the  eyes  have  been  removed. 

If  Planaria  is  captured  and  half  eaten  by  some  predaceous 
enemy,  it  is  not  a  very  serious  matter,  for  either  half  of  the 
body  is  able  to  regenerate  what  it  may  lack  and  after  a 
time  becomes  a  complete  individual  again.  During  such 
reconstruction  no  food  is  taken  as  a  rule,  but  the  body  is 
reorganized  and  forms  a  complete  new  worm  from  the  sub- 
stance already  within  it;  the  size,  of  course,  being  smaller. 
Some  species  of  planarians  can  regenerate  a  complete  body 
from  about  one  twenty-fifth  of  the  original,  but  there  is  a 
definite  limit  and  most  species  show  polarity  in  their  bodies. 
Polarity  in  this  sense  means  that  certain  regions  are  special- 
ized and  can  regenerate  only  particular  parts.  For  ex- 
ample, if  the  head  of  Planaria  is  cut  off  just  behind  the  eyes, 
the  anterior  piece  will  not  grow  a  new  body  but  grows 
another  head  and  becomes  a  two-headed  monster  (Fig.  73). 
A  small  portion  from  the  opposite  end  will  likewise  form  a 
double-tailed  individual.  Such  behavior  shows  that  the  two 
ends  of  the  body  are  so  specialized  that  they  can  only  form 
parts  like  those  they  already  possess. 

Race  Preservation. — Planaria  takes  advantage  of  its 
remarkable  powers  of  regeneration  to  produce  new  individ- 
uals. A  pair  of  eyes,  a  proboscis,  and  other  necessary 
organs  are  often  formed  in  the  posterior  half  of  body,  which 


PHYLUM  PLATYHELMIA  177 

then  breaks  away  from  the  anterior  portion.  New  in- 
dividuals are  thus  produced  by  transverse  fission. 

Planaria  also  carries  on  reproduction  by  sexual  processes 
involving  the  fusion  of  egg  and  sperm  cells.  Each  individual 
is  a  hermaphroditic  and,  therefore,  has  complete  sets  of  both 
male  and  female  organs  (i.e.,  testes,  vasa  deferentia,  or  sperm 
ducts,  penis;  and  ovaries,  oviducts,  sperm  receptacle,  yolk 
glands,  shell  glands) .  Though  a  planarian  might ' i  fertilize ' ' 
itself  by  injecting  its  own  spermatozoa  into  its  female  repro- 
ductive system,  it  does  not  commonly  do  so,  but  obtains  its 
supply  of  sperm  from  some  other  individual,  and  in  turn 
gives  its  sperm  to  fertilize  the  other  party  concerned  in  the 
mating.  The  eggs  laid  by  Planaria  are  of  two  types,  but 
both  have  several  yolk  cells  enclosed  in  the  shell  with  the 
ovum.  During  the  summer  thin-shelled  eggs  are  laid  which 
develop  quickly,  but  in  the  autumn  thick-shelled  "winter 
eggs"  are  produced  which  lie  dormant  until  spring. 

The  cleavage  and  gastrula  stages  in  Planaria' s  develop- 
ment are  somewhat  peculiar  but  there  is  general  agreement 
with  the  stages  usual  in  metazoans  (Fig.  64).  Though 
Planaria  is  like  a  gastrula  in  its  retention  of  the  single 
primitive  opening  into  the  enteron,  it  is  specialized  in  many 
other  respects.  There  is  a  large  mass  of  mesoderm  which 
consists  mostly  of  muscles  and  loose  parenchyma  cells 
(Fig.  76) ;  the  systems  of  organs  are  much  better  organized 
and  more  elaborate  than  in  ccelenterates,  particularly  the 
reproductive,  excretory  and  nervous  systems.  Specializa- 
tion toward  better  things  than  have  been  suggested  in  the 
structures  of  the  sponges  and  coelenterates  are  shown  by 
Planaria  (1)  in  the  assumption  of  the  bilateral  type  of 
symmetry,  and  (2)  in  cephalization.  Most  of  the  dominant 
and  highly  specialized  animals  on  the  earth  are  bilaterally 
symmetrical ;  in  fact,  radial  symmetry  may  usually  be  taken 
as  a  sign  of  racial  conservatism,  or  even  of  retrogression. 
Cephalization,  or  the  placing  of  important  organs  at  the 
anterior  end,  naturally  follows  the  acquisition  of  bilateral 
symmetry.  A  jellyfish  may  move  in  any  direction  and 


178  GENERAL  ZOOLOGY 

what  little  power  it  has  of  directing  its  attention  must  be 
divided  between  the  eight  or  more  marginal  sense  organs. 
A  planarian  has  only  one  chief  nerve  center  in  the  head; 
the  eyes  and  other  sense  organs  are  localized  at  the  anterior 
end.  The  jellyfish  can  perceive  stimuli  from  different 
directions;  the  planarian  always  moves  with  a  certain  part 
of  the  body  ahead  and  this  part  receives  most  of  the  stimuli 
from  the  outside  world.  Planaria  is  better  able  to  devote 
its  very  limited  powers  of  attention  to  one  thing  at  a  time 
because  the  chief  perceiving  and  modifying  organs  are 
gathered  together  to  some  extent,  and  from  a  psychological 
point  of  view  this  is  an  important  point.  The  ability  to 
pay  attention  comes  before  the  power  to  draw  conclusions, 
or  to  reason.  Though  Planaria,  on  account  of  its  bilateral 
symmetry  and  cephalization,  is  on  the  road  which  leads 
toward  the  higher  types  of  mental  development,  it  has  made 
little  progress.  Its  mental  powers  are  at  best  extremely 
limited,  and  its  activities  consist  largely  of  a  few  rather 
simple,  and  more  or  less  stereotyped,  reactions. 

There  are  a  number  of  different  kinds  of  turbellarians. 
Some  are  much  more  complicated  structurally  than  Planaria, 
and  others  are  far  simpler.  The  rhabdoco3les  usually  have  a 
straight  unbranched  digestive  cavity;  some  even  have  no 
definite  enteron  and  the  food  simply  enters  irregular  spaces 
in  the  parenchyma  after  passing  down  the  pharynx.  The 
polyclads  have  many  intestinal  branches  leading  out  from 
the  pharynx.  Planaria,  like  most  freshwater  turbellarians, 
belongs  with  the  triclads  because  its  enteron  has  three  chief 
branches. 

CLASS  2.    TREMATODA 

The  trematodes  are  all  parasitic  flatworms  with  a  cutic- 
ular  covering  and  a  rather  simple  enteron.  They  may  be 
divided  into  two  groups,  according  to  habits.  The  ecto- 
parasitic  trematodes  are  monogenetic,  that  is,  they  live  on 
the  outside  of  one  host.  They  have  a  simple  life  history, 
the  adults  laying  eggs  which  hatch  into  small  worms  like 


PHYLUM  PLATYHELMIA 


181 


found  in  the  intestine  of  the  animal  which  eats  the  herbi- 
vore. There  are  species  which  have  their  cycles  between: 
pig  and  man,  cow  and  man,  fish  and  man,  mealworm  and 
rat,  flea  and  dog,  rabbit  and  wolf,  etc. 

The  life  cycle  of  the  common  beef  tapeworm  is  shown  in 
Fig.  75.  An  adult  (A)  in  a  man's  intestine  is  continually 
giving  off  ripe  proglottids  (B)  which  pass  out  and  shed 
eggs  (C)  upon  the  ground.  A  single  proglottid  may 
produce  several  thousand  eggs,  and  the  contamination  of 


HOST 


FIG.  75. — Life  history  of  beef  tapeworm.  A,  adult  tapeworm  in  intestine  of 
man;  B,  proglottid  full  of  eggs  on  ground;  C,  eggs  on  ground;  D,  six-hooked 
larva  (onchophore)  set  free  and  bores  through  tissues  of  cow;  E,  cysticercus,  or 
bladderworm,  in  cow's  flesh;  F,  young  tapeworm  in  man. 

the  food  of  cattle  is  therefore  easily  accomplished.  A  ripe 
egg  contains  a  little  6-hooked  embryo  (D)  which  is  freed 
from  the  egg  shell  in  the  cow's  intestine  and  at  once 
bores  out  into  the  body.  In  the  liver,  or  the  muscles,  it 
comes  to  rest  and  undergoes  a  complete  reorganization. 
After  three  months  a  cysticercus,  or  bladder- worm  (E), 
has  developed  and  the  cow 's  flesh  may  then  infect  man  with 
tapeworms.  The  cysticercus  is  a  little  bladder  with  an  in- . 


182  GENERAL  ZOOLOGY 

verted  scolex  on  one  side.  If  it  gets  into  a  man's  intestine, 
the  scolex  is  pushed  out  (F),  fastens  its  suckers  on  the  wall 
of  the  intestine  and  soon  begins  to  bud  off  proglottids. 
About  1  per  cent,  of  all  the  cattle  slaughtered  in  the 
United  States  have  tapeworm  larvae  in  their  flesh,  and  there 
is  another  species  which  frequently  infests  pork.  Any  of 
these  larvae  will  be  killed  if  meat  is  thoroughly  cooked.  A 
piece  of  meat  if  boiled  should  be  cooked  about  fifteen  min- 
utes for  each  pound. 

PHYLUM  NEMERTINA 

The  nemerteans  resemble  the  flatworms  in  their  general 
form  but  are  more  specialized  in  many  respects :  an  anus  is 
present,  the  mouth  is  at  the  anterior  end,  there  is  a  retrac- 
tile finger-like  proboscis  above  the  mouth,  a  definite  cir- 
culatory system  is  present  for  carrying  blood.  Some 
nemerteans  are  of  large  size,  attaining  a  length  of  ninety 
feet.  Most  species  live  along  the  seashore,  but  a  few  are 
found  in  fresh  water  and  on  land. 


CHAPTER  XVII 
PHYLUM  NEMATOIDEA 

The  thread- worms,  or  round- worms,  are  slender  and 
cylindrical  in  form.  They  have  a  tubular  digestive  system 
beginning  with  a  mouth  and  opening  at  the  opposite  end 
through  an  anus.  There  is  a  body-cavity  between  the 
digestive  tube  and  the  body  wall  (Fig.  76).  Different 
species  of  round-worms  vary  greatly  in  habits :  some  are  free 
living,  others  are  scavengers,  and  many  live  as  parasites  in 
plants  or  animals.  There  are  three  classes: 


m 

-. ' 

fi 
ft  n 

A 
FIG.  76. — Cross-sections  of  a  flatworm  (A)  and  a  round-worm  (B)  compared. 

Class  1.  Nematoda,  round-worms  with  a  well-developed  digestive 
system  and  a  rather  simple  life  history. 

Class  2.  Gordiacea,  horsehair  worms.  Parasitic  Nematoidea  with 
the  digestive  system  often  partly  degenerate;  life  history  very  complex 
and  requiring  two  or  three  hosts  for  its  completion. 

Class  3.  Acanthocephala,  hook-headed  worms.  Parasitic  worms 
without  a  digestive  system  and  with  a  proboscis  at  the  anterior  end  which 
is  armed  with  spines;  in  the  life  cycle  there  is  an  alternation  between 
two  hosts. 

CLASS  1.     NEMATODA 

Most  nematodes  are.  very  resistant  to  variations  in  the 
conditions  of  life,  hence  it  is  easy  for  them  to  live  in  all  sorts 

183 


184  GENERAL  ZOOLOGY 

of  odd  situations.  Many  may  become  parasites  if  there  is 
opportunity  to  do  so,  and  a  large  number  of  species  are 
permanently  parasitic.  As  an  example  of  the  group  a 
parasite  of  man  will  be  considered. 

Ascaris  Lumbricoides  Linnaeus 

Self -maintenance. — Ascaris  is  a  round-worm  nearly  a  foot 
long  which  lives  in  the  small  intestine  of  man.  It  obtains 
its  nourishment  from  the  partly  digested  food  of  its  host. 
As  long  as  the  man  eats,  the  worm  has  a  plentiful  supply. 
There  are  three  lobes  at  the  anterior  end  which  act  as  lips 
and  poke  food  into  the  mouth.  These  are  assisted  by  a 
muscular  pharynx  which  exerts  a  sucking  action  and  draws 
particles  inside  the  body.  Behind  the  pharynx  the  food 
traverses  a  straight  intestine,  where  it  is  digested  and 
absorbed.  Undigested  fragments  pass  out  through  the 
anus  near  the  posterior  end  of  the  body.  No  special  cir- 
culatory system  is  present,  but  the  absorbed  food  passes 
through  the  wall  of  the  intestine  into  the  body-cavity 
(Fig.  76,  B),  where  it  is  free  to  move  from  one  end  of  the 
body  to  the  other,  and  all  parts  are  thus  supplied.  Excre- 
tions are  collected  by  two  tubes  which  run  longitudinally  on 
either  side  of  the  body  (ex)  and  open  together  to  the  exterior 
through  a  small  pore  on  the  ventral  side  near  the  anterior 
end. 

Self -protection. — Ascaris  is  in  no  danger  from  predatory 
enemies,  for  it  rests  safe  inside  some  man.  It  is  not  digested 
by  the  strong  secretions  in  the  intestine,  as  the  cuticle  on 
the  outside  of  its  body  is  impervious  to  most  chemicals 
and  furnishes  adequate  protection.  If  the  man  containing 
an  ascarid  dies,  the  worm  leaves  the  intestine  and  is  often 
found,  on  post-mortem  examination,  in  the  stomach,  throat 
or  mouth.  If  the  worm  succeeds  in  escaping  from  the  body, 
it  is  able  to  live  for  some  time  and  may  perhaps  find  en- 
trance into  another  host.  As  a  rule,  however,  Ascaris  is  in 
no  danger  if  it  once  finds  a  host — but  is  able  to  pass  its  life 
in  quiet  and  seclusion. 


PHYLUM  NEMATOIDEA 


185 


Race  Preservation. — Ascaris  is  dioecious,  that  is,  the 
sexes  occur  in  separate  individuals.  Males  are  easily  dis- 
tinguished from  females  by  their  smaller  size,  by  the  coiling 
of  the  tail,  and  by  the  two  little  setae  which  project  just  in 
front  of  the  anal  opening  (Fig.  77).  A  female  has  a  pair  of 
long  tubular  ovaries  which  swing  freely  in  the  body-cavity 
and  occupy  a  large  part  of  its  space.  The  ends  of  the  tubes 
are  enlarged  to  form  a  pair  of  uteri  in  which  the  eggs  develop 
to  some  extent  before  passing  from  the  body.  The  males 
have  a  single  tubular  testis  which  opens  through  the  anus. 

The  eggs  are  fertilized  far  up  in  the- ovarian  tubules  and 
pass  down  into  the  uteri  where  they  have  the  shell  added. 


C  D          E          F         G          H  I  J 

FIG.  77. — Life  History  of  Ascaris.  A,  male;  B,  female;  ex,  excretory  pore; 
u,  female  genital  aperture;  C,  zygote;  D,  zygote  without  covering:  E-H, 
cleavage  stages;  I,  J,  embryos;  K,  young  Ascaris  emerging  from  egg  shell. 

A  female  Ascaris  is  continually  laying  eggs,  producing  as 
many  as  15,000  in  a  day.  A  large  number  of  reproductive 
products  is  commonly  associated  with  parasitic  life  and  is 
necessary  because  the  chances  of  finding  a  favorable  place 
for  development  are  small.  This  is,  however,  the  only 
respect  in  which  Ascaris  shows  adaptation  to  parasitism. 
The  eggs  pass  out  of  the  host  and  develop  into  larvae  in 
moist  earth  (Fig.  77,  C-K)  but  do  not  hatch  unless  they  get 
into  a  man  (K).  They  cannot  infect  until  they  have  been 
upon  the  ground  thirty  to  forty  days,  and  then  are  intro- 
duced into  the  human  body  on  polluted  vegetables,  in  water, 
or  from  soiled  hands.  After  gaining  entrance  the  worm 
hatches,  becomes  mature,  and  begins  to  lay  eggs  in  about  a 


186  GENERAL  ZOOLOGY 

month.  Recent  investigations  indicate  that  Ascaris  may 
perhaps  pass  through  larval  stages  in  rats  or  mice  before 
entering  man. 

Comparing  Ascaris  with  a  flat  worm,  it  shows  two  great 
structural  advances — a  body-cavity  and  an  anus.  The 
flatworm  must  ingest  and  egest  through  a  single  opening; 
the  round-worm  has  a  much  more  convenient  arrangement 
with  an  entrance  for  food  and  an  exit  for  faeces.  The  body 
cavity  is  not  a  true  ccelom,  like  that  in  the  earthworm  (Figs. 
16,  89)  and  other  animals,  but  is  sufficiently  developed  to 
make  the  general  structure  of  Ascaris'  body  a  tube-within- 
a-tube,  as  described  in  Chapter  II.  It  is  simply  a  space 
between  the  muscles  of  the  body  wall  (Fig.  76,  B,  ra)  and 
the  digestive  tube.  This  space  is  a  great  convenience,  for 
it  allows  fluids  to  flow  readily  from  one  part  of  the  body  to 
another  without  having  to  pass  from  cell  to  cell,  as  is  the 
case  in  sponges,  ccelenterates,  and  also  to  some  extent  in 
flatworms. 

OTHER  NEMATODES 

An  Ascaris  living  in  a  man  usually  causes  no  very  serious 
trouble,  but  sometimes  one  will  crawl  up  the  bile  ducts  into 
the  liver  and  give  rise  to  an  abscess,  or  lodge  in  the  appendix 
and  induce  appendicitis.  Some  other  round-worms  which 
live  in  man  are  much  more  injurious.  The  hook-worm, 
Necator  americanus,  is  prevalent  throughout  the  south- 
eastern United  States,  where  it  was  probably  introduced 
by  slaves  brought  from  Africa.  It  gnaws  the  inner  wall  of 
the  intestine  and  secretes  toxins  which  impoverish  the 
blood  and  thus  weaken  the  body.  The  hook-worm  has  a 
peculiar  life  history — the  adults  lay  eggs  in  the  intestine; 
the  eggs  hatch  into  larvae  which  live  for  a  time  in  excre- 
ment or  in  the  soil,  and  when  grown  enter  man  by  boring 
through  the  skin;  they  then  get  into  the  blood  and  finally 
gain  entrance  to  the  alimentary  canal  by  way  of  the  lungs, 
trachea,  and  larynx.  Persons  with  a  heavy  hook-worm  in- 
fection may  be  abnormal  in  many  ways:  stunted  and  weak; 


PHYLUM  NEMATOIDEA  187 

beard  poorly  developed;  perverted  appetite  for  clay,  dirt, 
etc.;  dropsical  condition,  and  other  peculiarities.  The 
Rockefeller  Sanitary  Commission  has  agents  throughout  the 
southern  states  who  are  trying  to  better  conditions  among 
the  poorer  people  by  treatment  and  by  instruction  as  to 
proper  methods  of  living.  In  many  districts  the  shiftless- 
ness  of  the  population  is  largely  due  to  the  prevalence  of 
hook-worm.  Ninety  per  cent,  of  the  people  in  some 
counties  in  Louisiana  have  this  parasite.  I 

Another  very  injurious  round-worm  is  trichina,  Trichinella 
spiralis,  which  lives  in  rats,  hogs,  men,  and  a  few  other 
animals.  This  parasite  may  be  acquired  only  by  eating 
flesh  containing  the  encysted  larvae,  but  if  meat  is 
thoroughly  cooked  there  is  no  danger  of  infection.  Other 
important  human  parasites  are  the  pin-worm  and  the  whip- 
worm,  which  live  in  the  intestine;  and  the  filaria  worms 
which  inhabit  the  blood  and  lymph  vessels.  The  latter 
cause  a  horrible  disease  known  as  elephantiasis,  in  which 
the  legs  and  other  parts  of  the  human  body  become  greatly 
enlarged.  They  occur  only  in  tropical  countries,  and  are 
transmitted  by  the  bites  of  certain  mosquitoes.  There  are 
many  round-worms  which  cause  diseases  in  animals  and 
plants.  Tylenchus  tritid  is  an  important  plant  parasite 
which  produces  gall-like  growths  on  the  roots  of  tomatoes 
and  other  plants. 

CLASS  2.     GORDIACEA 

These  worms  (Fig.  78,  A)  are  commonly  known  as  horse- 
hairs, horsehair  worms,  and  horsehair  snakes,  on  account  of 
their  hair-like  form,  and  there  is  a  belief  among  ignorant 
persons  that  horsehairs  will  turn  into  worms  if  kept  in 
water.  They  are  often  very  numerous  along  the  shores  of 
ponds  and  streams  during  their  breeding  season.  At  such 
times  they  wrap  about  each  other  in  great  bunches,  forming 
veritable  "Gordian  knots "  (whence  the  name  of  the  class). 
Sometimes  the  adult  worms  get  into  man  and  live  for  a  time 
as  parasites. 


188  GENERAL  ZOOLOGY 

Most  Gordiacea  spend  the  greater  part  of  their  lives  as 
parasites  in  insects  and  often  have  very  complicated  life 
cycles.  Parachordodes  tolosanus  has  two  larval  stages :  the 
first  enters  the  aquatic  larva  of  the  orl-fly  and  lives  through 
the  winter;  the  second  develops  in  a  beetle  which  has 
eaten  an  adult  orl-fly  and  passes  another  winter  in  the 
beetle.  The  adult  lives  in  the  water,  as  a  parasite  in 
man,  or  in  some  other  animal. 

CLASS  3.     ACANTHOCEPHALA 

The  hook-headed  worms  (Fig.  78,  B)  are  parasitic  in 
fishes  and  other  vertebrates,  sometimes  even  in  man.  They 


B 

A 

FIG.  78. — A,  a  horsehair   worm,  Gordius;  B,   a  hook-headed   worm 
( Acanthocephala) . 

have  no  digestive  system  but  live  attached  to  the  wall  of 
the  intestine  like  a  tapeworm.  At  the  anterior  end  there  is 
a  protrusible  proboscis  which  is  armed  with  hooks  and 
serves  as  an  organ  of  attachment. 

The  development  of  acanthocephalans  requires  an  alter- 
nation of -hosts;  the  larva  usually  living  in  some  insect  and 
the  adult  in  a  vertebrate.  The  common  June-bug,  or  June- 
beetle,  carries  the  larva  of  Gigantorhynchus  suis.  This 
parasite  lives  as  an  adult  worm  in  the  intestines  of  pigs, 
which  become  infected  by  eating  beetles.  In  South 
Russia,  where  beetles  are  eaten  raw,  the  worm  is  said  to 
occur  in  man. 

PHYLUM  ROTIFERA 

On  account  of  the  ciliated  discs  at  the  anterior  end, 
rotifers  (Fig.  79),  were  called  " wheel-animalcules'7  by 
the  zoologists  who  first  studied  them.  When  the  cilia  are 


PHYLUM  ROTIPERA 


189 


in  motion  they  make  the  discs  appear  like  rotating  wheels 
— hence  the  name  Rotifera  (fero,  I  carry;  rota,  a  wheel). 

These  animals  are  remarkable  for  their  small  size.  Though 
they  are  well-developed  metazoans  with  more  special- 
ized systems  of  organs  than  round-worms,  they  are  usually 
no  larger  than  single-celled  animals,  and  most  species  can 
barely  be  seen  with  the  naked  eye.  The  mouth  is  situ- 
ated between  the  trochal  (ciliated)  discs  where  it  receives 
the  food  brought  in  the  vortex  created  by  the  cilia.  The 
food  is  ground  in  a  mastax,  or  chewing  stomach,  which  is 


FIG.  79.— Rotifera. 

provided  with  chitinous  teeth.  Behind  the  mastax  there 
is  a  glandular  stomach  where  digestion  takes  place,  and  the 
residue  passes  out  through  an  anus  on  the  dorsal  side  near 
the  posterior  end.  The  excretory  system  consists  of  a 
pair  of  branched  tubules  and  a  contractile  bladder  which 
empties  through  the  anus.  Rotifers  have  a  body  cavity, 
but  it  is  not  a  true  ccelom. 

Rotifers  are  of  separate  sexes.  The  males  are  smaller  than 
the  females  and  are  often  degenerate — sometimes  the 
digestive  system  is  entirely  lacking.  The  females  produce 
two  kinds  of  eggs:  summer  and  winter.  The  summer  eggs 
are  parthenogerietic,  have  thin  shells,  and  are  of  two  sizes — 
the  larger  producing  females  and  the  smaller  males.  The 


190  GENERAL  ZOOLOGY 

winter  eggs,  which  are  fertilized,  have  thick  shells,  are 
able  to  pass  through  a  long  resting  period  before  hatching, 
and  always  develop  into  females. 

The  ordinary  life  of  a  rotifer  is  about  six  weeks  but  certain 
species,  if  dried  slowly,  will  form  gelatinous  cysts  and  are 
then  able  to  live  in  a  dormant  condition  for  a  long  time. 
When  encysted,  rotifers  can  stand  great  extremes  of  tem- 
perature without  being  killed. 

Rotifers  in  their  adult  condition  resemble  a  trochophore 
larva,  which  occurs  in  the  development  of  certain  molluscs, 
annelids,  and  in  some  other  animals.  This  has  led  some 
zoologists  to  believe  that  rotifers  are  closely  related  to  the 
ancestral  forms  of  molluscs,  worms,  etc.,  but  such  relation- 
ship cannot  be  very  close  because  the  rotifers  are  a  very 
specialized  group,  and  would  not,  therefore,  give  rise  to 
new  lines  of  descent. 

PHYLUM  BRACHIOPODA 

Brachiopods  are  marine  animals  having  a  calcareous 
bivalve  shell  and  usually  a  short  stalk  which  fastens  them 
to  the  ocean  bottom.  They  resemble  clams  somewhat  in 
appearance  but  have  no  close  relationship.  The  valves 
of  a  brachiopod's  shell  are  dorsal  and  ventral,  instead  of 
right  and  left  as  in  clams.  The  body  is  very  small  and  the 
space  within  the  shell  is  mostly  filled  by  two  ciliated  arms 
which  create  currents  for  procuring  food.  Within  the  body 
there  is  a  spacious  coelom,  a  stomach,  heart,  and  digestive 
gland.  An  anus  may  be  present  or  absent.  All  the  struc- 
tures mentioned  show  some  resemblance  to  those  of  other 
worm-like  forms,  but  are  modified  to  suit  a  sessile  life. 

Brachiopods  lead  a  sluggish  existence  at  the  bottom  of 
the  ocean,  and  have  done  so  since  the  earliest  geological 
times.  Most  of  the  shells  which  appear  as  fossils  are  the 
remains  of  these  animals.  The  abundance  of  such  deposits 
indicates  the  past  dominance  of  the  Brachiopoda,  which 
at  one  time  shared  the  earth  with  trilobites  and  archaic 


PHYLUM  PRYOZOA  191 

molluscs — when  there  were  no  vertebrates  to  interfere  with 
their  simple  activities. 

PHYLUM  BRYOZOA 

Bryozoans,  or  "moss-animals"  are  usually  colonial  in 
habit  and  resemble  hydroids  in  general  appearance.  Most 
species  are  marine  but  a  few  occur  in  fresh  water.  They 
appear  as  encrustations,  or  as  little-  plant-like  growths, 
attached  to  objects  in  the  water.  The  individuals  of  a 
colony  are  called  zooids.  Each  has  a  circle  of  tentacles 
about  the  mouth  and  shows  a  superficial  radial  symmetry, 
as  is  the  case  in  many  attached  animals.  The  digestive 
tube  is  U-shaped  and  the  anus  therefore  opens  near  the 
mouth.  Despite  their  superficial  resemblance  to  hydroids, 
bryozoans  are  more  nearly  related  to  the  worms.  They, 
like  the  brachiopods,  are  peculiar  on  account  of  their  seden- 
tary existence. 


CHAPTER  XVIII 
PHYLUM  ECHINODERMATA 

The  name  of  this  phylum  means  "  spiny  skin,"  and  is 
fitting  for  most  representatives.  Echinoderms  are  radially 
symmetrical,  and  usually  have  five  antimeres;  an  anus  is 
present  in  some  but  non-functional  or  absent  in  others ;  the 
ccelom  is  large  and  well  developed.  The  distinctive  feature 
of  echinoderm  structure  is  the  ambulacral,  or  water-vas- 
cular, system  which  serves  primarily  for  locomotion.  There 
are  five  classes:  (1)  Asteroidea,  starfishes;  (2)  Ophiuroidea, 
brittle-stars,  or  snake-stars;  (3)  Echinoidea,  sea-urchins; 
(4)  Holothuroidea,  sea-cucumbers;  (5)  Crinoidea,  stone- 
lilies. 

CLASS  1.     ASTEROIDEA 

The  starfishes  all  have  the  chief  axis  short,  and  the  dis- 
tance from  the  mouth  to  the  opposite  pole  of  the  body  is 
accordingly  abbreviated.  The  central  portion  of  the  body, 
or  disc,  is  not  sharply  set  off  from  the  arms,  but  grades 
rather  gradually  into  them.  Most  starfishes  have  five  arms, 
or  rays,  but  there  are  several  species  which  have  more;  the 
number  sometimes  reaching  as  high  as  forty.  The  common 
starfish  along  the  seashore  on  the  Atlantic  coast  of  the 
United  States  is  Asterias  forbesi  (Desor)  Verrill,  and  it  will 
be  discussed  in  detail  as  an  example  of  the  class. 

'   THE  STARFISH 

Self-maintenance. — Asterias  (Fig.  80)  moves  about  by 
means  of  the  tube-feet,  of  which  there  are  four  rows  extend- 
ing from  a  longitudinal  groove  along  the  underside  of  each 
arm.  A  tube-foot  has  an  adhesive  disc  at  its  tip  and  is  con- 

192 


PHYLUM  ECHINODERMATA 


193 


nected  with  a  little  reservoir,  or  ampulla,  within  the  body 
wall.  Muscles  in  the  wall  of  the  tube-foot  make  it  possible 
to  move  the  organ  about,  like  an  elephant's  trunk,  and  the 
adhesive  disc  may  thus  be  put  down  in  a  particular  spot. 
When  the  tube-foot  is  extended  the  reservoir  connected 
with  its  base  is  small,  but  if  it  contracts,  the  reservoir  is 
distended  by  the  water  which  is  forced  into  the  body. 
Water  first  enters  the  starfishes'  water-vascular  system 
through  the  madreporic  plate,  or  " sieve  plate,"  passes 
through  a  straight  stone  canal  to  a  ring  canal  around  the 
mouth,  and  from  this  a  radial  vessel  which  supplies  the 


B  c 

FIG.  80. — Starfishes.     A,   eating   an   oyster;   B,    an  individual   which  has  lost 
four  rays  and  is  growing  them  anew;  C,  crawling  by  using  the  tube-feet. 

tube-feet  runs  down  the  under  side  of  each  arm.  The 
broad  spread  of  the  body  and  the  great  number  of  tube- 
feet  enable  starfishes  to  walk  over  soft  mud,  and  the  ad- 
hesive discs  permit  them  to  climb  up  vertical  surfaces  with 
ease.  Though  able  to  move  but  slowly,  starfishes  are  well 
equipped  to  seek  for  food. 

A  starfish  can  tell  when  substances  are  present  in  the 
water  which  indicate  that  food  is  near.     If  an  individual  is 


194  GENERAL  ZOOLOGY 

resting  quietly  in  a  current  and  clam  juice  is  added  to  the 
water,  it  will  become  restless  and  move  about.  If  a  screen 
is  placed  so  that  the  starfish  cannot  reach  the  food,  it  will 
move  as  near  as  possible  and  try  to  pass  by  the  obstruction. 

On  account  of  its  taste  for  molluscs,  the  starfish  is  one  of 
the  worst  enemies  of  the  oyster  and  clam  fishermen.  The 
method  by  which  a  starfish  opens  an  oyster  was  long  a  matter 
of  speculation, .  erroneous  observation,  and  dispute.  One 
man  thought  an  acid  was  secreted  which  dissolved  the 
shell;  another  asserted  that  the  starfish  took  the  oyster  by 
surprise  and  stuck  its  arm  in  the  gaping  shell  before  it 
could  be  closed;  other  equally  fantastic  theories  had  advo- 
cates. The  starfish  opens  oysters  by  tiring  them  out. 
The  tube-feet  are  attached  on  either  side  of  the  shell  and  a 
steady  pull  is  thus  brought  to  bear  on  opposite  sides  (Fig. 
80,  A).  The  oyster  has  two  strong  muscles  to  keep  the 
shell  closed,  but  these  finally  become  tired  and  the  starfish 
then  gains  access  to  the  food  inside. 

A  starfish  has  a  most  direct  route  from  food  to  stomach. 
After  an  oyster  has  been  opened,  soft  folds  of  the  stomach 
are  protruded  through  the  mouth  and  wrapped  about  the 
parts  suitable  for  food.  Digestion  and  absorption  take 
place  inside  the  oyster's  shell  and  outside  the  starfish's 
body!  There  is  one  disadvantage  connected  with  this 
method  of  feeding — large  amounts  of  digestive  fluids  are 
wasted.  A  starfish  therefore  has  enormous  digestive  glands 
which  fill  most  of  the  space  within  the  body.  The  anus  in 
most  starfishes  is  not  functional,  and,  indeed,  it  has  small 
chance  to  be,  for  waste  seldom  enters  the  short  intestine. 

About  the  big  baggy  stomach  there  is  a  spacious  ccelom 
and  in  this  cavity  the  food  may  pass  readily  to  all  parts  of 
the  body.  A  set  of  tubes  (hsemocoel)  also  carries  fluid  to 
certain  obscure  regions,  but  the  starfish  has  little  need  of 
a  separate  circulatory  system.  Respiration  takes  place 
through  the  tube-feet  and  branchiae.  The  former  are 
admirably  fitted  for  respiratory  purposes;  the  reservoirs,  or 
ampullae,  being  in  the  coelomic  cavity,  and  the  external  por- 


PHYLUM  ECHINODERMATA  195 

tions  with  their  thin  walls  swinging  about  freely  in  the 
water.  The  branchiae  are  minute,  finger-like  organs  which 
are  scattered  plentifully  over  the  whole  upper  (aboral) 
surface  of  the  starfish.  They  are  hollow  and  connect 
directly  with  the  coelom.  Cilia  keep  currents  passing  in  and 
out  of  the  branchiae,  and  the  coelomic  fluid  is  thus  aerated 
through  the  thin  membranes  exposed  to  the  sea  water. 
Asterias  has  no  special  means  of  excretion,  except  little 
amoeboid  cells*  which  wander  about  in  the  tissues  engulfing 
waste  products.  These  finally  migrate  outside  the  body 
with  their  loads. 

Self-protection. — Asterias  is  protected  from  most  prer 
daceous  animals  by  the  hard,  spiny  skeleton.  When  left 
stranded  at  low  tide,  however,  its  watery  body  readily 
dries  up.  A  starfish  crawls  boldly  over  the  rocks  at  high 
tide,  but  creeps  into  crevices  and  hides  when  the  ocean 
ebbs.  There  is  a  simple  eye  at  the  tip  of  each  ray  and  the 
whole  surface  of  the  body  is  more  or  less  sensitive  to  light ; 
so  Asterias  can  tell  when  it  is  in  light  or  shadow,  and  is 
able  to  crawl  toward  dark  spaces. 

A  starfish  moves  very  slowly  and  the  body  gathers  many 
particles  of  dirt  which  fall  from  above.  Such  accumula- 
tions are  continually  swept  to  the  edge  of  the  body  and  off 
by  the  cilia  which  cover  the  soft  parts  between  the  spines. 
There  are  many  very  small  animals  in  the  ocean  which 
might  gnaw  the  branchiaB  and  the  flesh  of  the  starfish  if  it 
were  not  for  the  presence  of  little  seizing  organs,  the 
pedicellarise,  which  are  to  be  found  around  the  bases  of  the 
spines.  Each  pedicellaria  consists  of  a  short  stalk  sur- 
mounted by  a  pair  of  calcareous  pinchers.  If  a  small 
animal  starts  to  walk  over  the  starfish,  it  is  seized  and  held 
immobile  until  it  dies  (Fig.  80,  C).  The  pedicellariae  are 
rather  independent  in  reacting  and  will  often  pull  against 
each  other.  They  grasp  anything  that  moves  and  hold  on 
until  they  are  jerked  loose  from  the  body,  or  until  the  thing 
they  hold  ceases  to  move.  Through  their  activities  the 


196 


GENERAL  ZOOLOGY 


soft  parts  of  the  body  are  shielded  from  small  predaceous 
animals. 

If  a  starfish  meets  with  an  accident  and  has  one  of  its 
arms  hurt,  the  injured  appendage  is  cast  off  near  its  base  and, 
after  a  time,  a  new  one  grows  out.  The  body  may  be  cut 
in  two  and  each  of  the  parts  can  regenerate  what  it  lacks. 
By  careful  experiments  it  has  been  found  that  Asterias  for- 
besi  is  able  to  form  a  complete  body  if  left  with  the  disc  and 
one  arm  (Fig.  80,  B).  Professor  Mead  kept  a  separate 
arm  alive  for  three  months,  but  it  did  not  regenerate  any- 
thing, although  it  was  apparently  in  good  condition  and 
crept  about  actively.  Some  years  ago  starfishes  were  com- 
monly dredged  up,  cut  in  two,  and  thrown  back  into  the 


FIG.  81. — Development  of  starfish.  A,  egg  and  sperm  cells;  B,  zygote; 
C— F,  cleavage  stages;  F,  morula;  G,  section  through  gastrula  stage;  H,  ciliated 
bilaterally  symmetrical  larva;  a,  anus;  m,  mouth;  I,  young  starfish  growing  out 
from  side  of  bilaterally  symmetrical  larva. 

ocean.  The  oyster  fisherman  supposed  they  were  killed 
by  such  treatment,  but  it  of  course  only  increased  the  num- 
ber of  the  pests. 

Race  Preservation. — Starfishes  are  dioecious  animals  and 
both  the  males  and  females  shed  their  ripe  sexual  cells  into 
the  ocean.  The  sperm  cells  swim  to  the  ova,  fertilize  them, 
and  development  passes  through  the  stages  characteristic 
of  all  metazoans  (Fig.  81,  A-G).  After  gastrulation  (G) 
a  bilaterally  symmetrical  larva  is  formed,  which  is  known 
as  a  brachiolaria  (H).  It  has  a  mouth  (m),  digestive  tube, 


PHYLUM  ECHINODERMATA  197 

anus  (a),  and  ccelom.  After  swimming  about  for  a  day  or 
two  the  brachiolaria  attaches  itself  to  a  bit  of  seaweed  and 
five  little  arms  grow  out  on  one  side,  thus  beginning  the 
development  of  the  radially  symmetrical  starfish  (I) .  Most 
of  the  brachiolaria  collapses  and  in  part  disintegrates; 
the  young  starfish  soon  starts  out  to  hunt  clams  and  oysters. 
The  growth  of  a  starfish  depends  upon  the  amount  of  food 
it  is  able  to  secure.  Mead  separated  two  which  changed 
from  larvae  at  the  same  time  and  kept  them  under  obser- 
vation for  thirty-nine  days.  One  was  fed,  the  other  received 
no  food,  and  at  the  end  of  the  period  the  former  was 
fifty  times  as  large  as  the  latter.  A  starfish  may  become 
mature  in  less  than  a  year;  and,  as  each  female  lays  several 
thousands  of  eggs,  the  rate  of  increase  is  rapid. 

Although  Asterias,  like  most  starfishes,  uses  only  sexual 
methods  of  reproduction  to  produce  new  individuals,  there 
are  some  species  which  are  able  to  multiply  asexually. 
Linckia,  for  example,  breaks  off  the  tips  of  its  arms  and  such 
fragments  grow  into  complete  new  individuals. 

GENERAL  REMARKS  ON  STARFISHES 

The  starfish  is  a  fairly  successful  representative  of  a 
somewhat  unprogressive  biological  principle.  It  has  greatly 
elaborated  the  radial  type  of  symmetry,  and  represents, 
with  other  echinoderms,  the  highest  degree  of  speciali- 
zation along  such  lines.  That  the  radial  arrangement  of 
body  parts  has  some  things  in  its  favor  cannot  be  disputed. 
It  is  certainly  advantageous  to  be  able  to  move  with  equal 
facility  in  any  direction.  It  is  desirable  to  be  able  to 
take  food  or  receive  notice  of  danger  on  any  side  of  the 
body. 

But  usually  the  disadvantages  more  than  compensate  for 
the  favorable  features  associated  with  radial  symmetry. 
As  was  pointed  out  in  connection  with  the  coelenterates, 
it  is  hard  for  a  radially  symmetrical  animal  to  correlate  its 
abilities  for  a  particular  end.  A  starfish  has  five  arms 


198  GENERAL  ZOOLOGY 

bearing  five  equally  important  sets  of  sense  organs  (eyes, 
tactile  tentacles,  etc.)  and  what  little  power  of  attention 
it  has  must  vacillate  between  these  or  be  continually  divided. 
This  is  important  because  the  development  of  higher 
psychical  processes  (memory,  power  of  attention,  reason, 
etc.,  as  they  appear  in  animals  with  well-centralized  nervous 
systems)  depends  upon  seriously  attending  to  one  thing 
at  a  time. 

The  behavior  of  a  starfish  is  largely  made  up  of  more 
or  less  automatic  " reflexes."  If  an  individual  is  turned 
with  its  mouth  uppermost,  it  slowly  pulls  itself  over  again, 
by  using  the  tube-feet.  Observers  have  noticed  that 
various  individuals  may  have  different  methods  of  righting 
themselves — that  is,  they  have  different  " habits"  of  doing 
the  same  sort  of  work,  and,  therefore,  show  some  individ- 
uality. Professor  Jennings  induced  starfishes  to  forsake 
their  usual  methods  of  turning  over,  and  taught  them  new 
ways.  For  example,  a  certain  individual  always  began  to 
turn  with  a  particular  arm,  and  when  prevented  from  using 
that,  learned  to  begin  with  another.  Such  lessons  are  very 
simple,  but  they  show  the  limits  of  the  starfish's  "mind." 

A  starfish  has  its  nervous  system  separated  into  three 
divisions;  all  essentially  alike  in  structure.  There  are 
five  nerves  running  longitudinally  in  each  of  the  arms,  and 
these  are  connected  in  the  disc.  Two  divisions  are  near 
the  oral  surface,  and  the  third  lies  in  the  aboral  body  wall. 
If  the  central  connections  are  cut,  the  arms  no  longer 
work  together,  but  e.ach  can  still  carry  on  its  own  activities. 
If  an  individual  in  which  the  connecting  nerves  have  been 
severed  is  turned  over,  each  of  its  arms  tries  to  get  back  by 
itself.  So  far  as  its  psychology  is  concerned,  a  starfish 
consists  of  five  centers,  or  individuals,  which  usually  work 
in  harmony  through  the  influence  of  the  central  connections. 

The  behavior  made  necessary  by  radial  symmetry  has, 
then,  made  the  starfish  unprogressive,  and  generally  diffuse 
psychologically.  And,  if  we  look  at  the  starfishes'  ances- 
tral tree,  there  is  evidence  that  there  has  been  retro- 


PHYLUM  ECHINODERMATA  199 

gression.  The  embryological  evidence  (Fig.  81)  indicates, 
according  to  the  Law  of  Biogenesis,  that  starfishes  were 
once  free  swimming  bilaterally  symmetrical  animals  (Fig. 

81,  H),  with  all  the  possibilities  that  go  with  that  type 
of   bodily  arrangement,   and   that   they  gave  up  certain 
possibilities  to  become  radially  symmetrical  and  unprogress- 
ive.     It  is  believed  that  this  change  came  about  because 
echinoderms  at  one  time  in  their  racial  history  took  up 
a  sessile  mode  of  life  and  grew  attached  to  the  ocean  bottom. 
It  is  desirable  for  an  attached  animal  to  be  able  to  receive 
stimuli. and  capture  food  from  as  many  directions  as  possible, 
hence  the  assumption  of  radial  symmetry.     But  echino- 
derms, as  a  race,  made  a  serious  mistake  when  in  the 
course  of  evolution  they  came  to  an  attached  mode  of 
existence,    for    no  sessile    animal  can  progress  very  far. 
There  are  now  some  points  of  actual  degeneracy.     For  ex- 
ample, the  anus  of  the  larva  is  functional  (and  was  probably 
so  in  the  ancestral  types) ,  but  in  many  adult  echinoderms  is 
not  used  or  is  absent.     In  modern  times  there  have  been 
evolutionary    attempts    to    correct    early    racial    errors: 
most  echinoderms,  like  the  starfish,  have  taken  up  a  free- 
living  existence;  some  sea-urchins  and  sea-cucumbers  have 
even    become    bilaterally    symmetrical    as    adults    (Fig. 

82,  B,  F). 

CLASS  2.     OPHIUROIDEA 

The  brittle-stars,  or  snake-stars  (Fig.  82,  C)  are  very 
active,  being  able  to  bend  the  solid  arms  readily  and  thus 
swim  about.  They  feed  mostly  on  minute  organisms 
which  they  scrape  from  marine  plants  with  their  feet. 
No  anus  is  present. 

On  account  of  their  activity,  ophiuroids  have  been  much 
used  for  experimental  work.  Attempts  to  teach  them 
simple  lessons  have  met  with  little  success.  In  one  series 
of  experiments  a  rubber  tube  was  slipped  over  one  of  an 
ophiuroid's  arms,  and  behavior  in  attempting  to  remove  it 
observed.  There  was  "  neither  a  decrease  in  the  amount 


200 


GENERAL  ZOOLOGY 


of  time  taken  to  solve  the  problem,  nor  an  increase  in  the 
frequency  of  movements  best  fitted  to  solve  it,"  hence  no 
learning. 

Another  line  of  experimental  work  related  to  the  regenera- 
tion of  lost  parts.  Ophiuroids  readily  break  off  pieces  of 
their  arms  (whence  the  name,  brittle-star)  and  many 
species  commonly  practise  autotomy  to  escape  from  ene- 
mies. If  an  arm  is  grasped,  it  is  thrown  off  and  left 
to  wriggle  in  the  claws  of  the  aggressor,  while  the  ophiuroid 
escapes.  Missing  parts  are  readily  regenerated  and  this 


A  B          C  D  £  F 

FIG.  82. — Echinodermata.  A,  crinoid;  B,  heart  urchin;  C,  brittle  star; 
D,  regular  sea  urchin;  E,  sea  cucumber  buried  in  mud;  F,  cake  urchin,  or  "sand 
dollar." 

was  made  the  basis  for  experimental  work.  One  investi- 
gator maintained  that  the  rate  of  regeneration  was  propor- 
tional to  the  degree  of  injury  (i.e.,  the  more  cut  off,  the  faster 
the  growth  to  replace  the  lost  part),  but  further  experiments 
showed  that  such  a  ratio  would  not  hold  absolutely. 

CLASS  3.     ECHINOIDEA 

The  sea-urchins  differ  from  all  other  echinoderms  in 
possessing  (1)  an  immovable  calcareous  test,  which  is 


PHYLUM  ECHINODERMATA  201 

usually  more  or  less  obscured  by  tube  feet  and  spines,  and 
(2)  by  the  presence  of  a  complicated  system  of  calcareous 
ossicles  which  make  up  a  chewing  apparatus  known  as 
" Aristotle's  lantern."  Sea-urchins  are  globular,  discoidal, 
or  heart-shaped,  and  at  first  glance  have  little  in  common 
with  the  starfish,  but,  if  one  could  take  a  starfish  and  bend 
the  arms  upward  so  that  their  ends  came  together  and  then 
fuse  the  sides,  he  would  have  made  a  sea-urchin.  The  cal- 
careous tests  of  sea-urchins  make  respiration  difficult  and 
many  species  have  therefore  developed  little  brush-like 
gills  about  the  mouth. 

The  cake-urchins  and  heart-urchins  have  the  same  general 
plan  of  structure  as  the/ 'regular"  sea-urchins,  but  have  sec- 
ondarily assumed  bilateral  symmetry.  The  cake-urchins, 
or  sand  dollars  (Fig.  82,  F),  have  the  anus  at  one  side  (in- 
stead of  at  the  pole  opposite  the  mouth  as  in  regular  forms) , 
and  are  very  flat,  so  that  they  may  move  about  on  a  soft 
bottom  without  sinking  in.  The  heart-urchins  have  the 
mouth  at  one  side  and  the  anal  opening  at  the  other  (B). 

CLASS  4.     HOLOTHUROIDEA 

In  the  sea-cucumbers  (Fig.  82,  E)  the  calcareous  exoskele- 
ton,  so  characteristic  of  other  echinoderms,  is  extremely 
degenerate,  being  reduced  to  little  spicules  which  are  em- 
bedded in  the  leathery  skin.  The  chief  axis  is  long,  so 
that  the  body  is  worm-like.  There  is  a  ring  of  tentacles 
for  feeding  about  the  mouth.  Respiration  usually  takes 
place  through  the  anus;  water  being  pumped  by  muscular 
action  into  two  great  respiratory  trees,  which  branch  out 
among  the  internal  organs  and  fill  up  a  large  part  of  the 
space  within  the  body. 

Sea-cucumbers  show  remarkable  instances  of  autotomy, 
or  the  casting  off  of  parts  of  the  body.  One  species,  if  re- 
moved from  the  sand  in  which  it  is  accustomed  to  burrow, 
will  break  off  pieces  from  its  posterior  end  until  only  a  twen- 
tieth of  the  original  body  remains.  Another  species  when 


202  GENERAL  ZOOLOGY 

placed  in  stagnant  water,  will  throw  out  almost  all  of  its 
internal  organs  (entire  alimentary  canal,  most  of  the  nervous 
and  water- vascular  systems),  in  fact,  everything  except  the 
body  wall  and  the  respiratory  tree.  Certain  tropical  species 
when  taken  in  the  hands  will  turn  into  a  slippery  mass  of 
slime  which  cannot  be  held.  Many  of  these  sea-cucumbers 
can,  however,  if  conditions  become  favorable,  regenerate  all 
their  missing  parts  in  a  short  time. 

CLASS  5.     CRINOIDEA 

The  crinoids,  or  stone-lilies  (Fig.  82,  A),  live  for  the  most 
part  at  considerable  depths  in  the  ocean.  They  have  five 
branched  arms  which  are  covered  with  little  tentacle-like 
tube-feet.  All  crinoids  are  attached  by  the  aboral  pole 
when  young,  but  some  species  break  loose  from  the  stalk 
when  mature  and  swim  about  by  waving  their  arms.  The 
permanently  stalked  species  often  attain  a  great  length, 
some  measuring  thirty  or  forty  feet.  Stone-lilies  flour- 
ished in  great  numbers  during  past  geological  ages,  and  lime- 
stone deposits  are  often  filled  with  their  remains,  the  most 
common  objects  being  little  calcareous  discs  from  the  stalks. 


CHAPTER  XIX 
PHYLUM  ANNELIDA 

The  annelids  are  more  completely  metameric  than  the 
animals  in  any  other  phylum.  The  flatworms  and  nema- 
todes  do  not  have  any  indication  of  segmentation  in  their 
bodies,  the  rotifers  are  only  imperfectly  segmented,  but  the 
annelids  are  always  made  up  of  a  chain  of  very  similar 
parts.  The  phyla  which  rank  above  the  annelids  in 
structural  complexity  have  the  metamerism  obscured  by 
the  outgrowth  of  the  limbs  and  other  appendages  (Arthro- 
poda,  Chordata),  or  lack  any  trace  of  segmentation  (Mol- 
lusca).  This  phylum  therefore  shows  metamerism  as  it 
appears  nowhere  else  in  the  animal  kingdom.  As  a  rule 
the  ccelom  of  annelids  is  large  and  well  developed;  the  body 
is  therefore  of  the  tube-within-a-tube  type  (Fig.  16).  The 
alimentary  canal  possesses  two  openings.  Annelids  live 
in  the  ocean,  in  fresh  water,  and  on  land;  some  are  even 
parasitic  upon  other  animals.  There  are  two  important 
classes  and  one  or  two  of  less  general  interest,  which  will 
not  be  considered. 

Class  1.  Choetopoda.  Annelids  with  setae,  or  bristles,  along  the 
sides  of  the  body. 

Class  2.  Hirudinea.  Without  setae,  but  with  suckers  at  either  end 
of  the  body. 

CLASS  CILETOPODA 

The  chsetopods  (chaite,  bristle;  pous,  foot)  are  abundant 
in  most  parts  of  the  earth  and  are  easily  recognized  by  the 
setae  along  the  sides  of  the  body.  Sometimes  the  setse  pro- 
trude in  great  brush-like  tufts  from  each  metamere,  in 
other  instances  they  are  small  and  almost  completely  buried 

203 


204 


GENERAL  ZOOLOGY 


in  the  body  wall.  They  usually  take  the  form  of  stiff  horny 
bristles  which  serve  for  locomotion.  The  earthworms  are 
the  most  familiar  representatives  of  the  class,  and  one  of  the 
commoner  species  will  therefore  be  considered  as  an  example. 


THE  EARTHWORM,  Lumbricus  hercukus  Savigny 

This  is  the  large  earthworm  found  burrowing  in  humus 
soil  throughout  the  northeastern  United  States  (Fig.  83). 
It  sometimes  reaches  a  length  of  ten  or  twelve  inches,  and 
is  most  often  seen  above  ground  after  rains. 

Self -maintenance. — Lumbricus  eats  its  way  through  the 
ground  and  thus  obtains  considerable  food  from  the  organic 
matter  present.  Most  of  the  material  excavated  passes 


A  .* 


FIG.  83. — Earthworms.     A,  castings;  B,  mating;  C,  egg  cocoon  in  ground;  D, 
crawling  in  burrow;  E,  pulling  a  leaf  into  burrow. 

through  the  alimentary  canal  and  is  voided  into  the  burrow 
behind  the  worm  or  taken  to  the  surface  at  night  and  left 
as  a  pile  of  castings  (Fig.  83,  A).  The  method  by  which  an 
earthworm  moves  through  its  burrow  is  of  interest.  The 
setae  and  two  muscle  layers  are  the  chief  agents  in  locomo- 
tion. If  an  earthworm  is  at  rest  and  begins  to  move  for- 
ward the  sequence  of  events  will  be  somewhat  as  follows: 
the  setae  at  the  posterior  end  of  the  body  are  protruded  and 
take  firm  hold  on  the  sides  of  the  burrow,  then  the  circular 
muscles  (running  around  the  body)  contract  to  make  the 
body  longer,  and  as  the  posterior  portion  is  anchored,  the 


PHYLUM  ANNELIDA  205 

front  end  then  moves  forward.  The  setae  at  the  front  end 
are  next  extended  as  those  of  the  posterior  end  £re  with- 
drawn and  the  longitudinal  muscles  by  contracting  pull  the 
body  forward.  Progress  is  continued  by  repeating  such 
activities. 

Though  the  earthworm  is  an  indiscriminate  feeder,  it  is 
not  without  a  discriminating  sense  of  taste.  At  night 
leaves  and  other  parts  of  plants  are  pulled  down  into  the 
burrows  (E),  to  be  eaten  later.  If  foods  such  as  cabbage, 
onion,  and  meat  are  placed  on  the  surface  of  the  ground 
where  worms  are  abundant  and  left  over  night,  it  will  be 
found  that  marked  preferences  are  shown.  Usually  the 
meat  will  be  eaten  first,  then  the  cabbage,  and  finally  the 
onion.  Hurwitz  has  recently  shown  that  the  chemical 
sense  in  the  earth  worm's  skin  is  more  acute  in  some  respects 
than  the  sense  of  taste  in  man.  He  suspended  worms  over 
different  solutions  so  that  they  could  be  dipped  in  quickly. 
By  varying  the  strength  of  the  solutions  and  recording  the 
time  it  took  worms  to  withdraw  from  them  a  rather  accu- 
rate test  was  made. 

The  alimentary  canal  of  the  earthworm  is  well  suited  to 
extract  the  small  amount  of  nutriment  from  the  rather 
indigestible  soil.  The  soft,  finger-like  prostomium  above 
the  mouth  continually  scrapes  away  particles  so  that  they 
may  be  sucked  inside  and  swallowed  by  the  muscular 
pharynx.  After  passing  the  pharynx  and  esophagus,  the 
food  is  held  in  the  large  thin-walled  crop  until  it  can  be 
received  for  grinding  in  the  tough  gizzard.  When  thor- 
oughly pulverized  and  mixed  the  mass  enters  the  intestine 
where  it  is  digested  with  glandular  secretions  and  absorbed. 
The  large  amount  of  sandy  grit  in  the  food,  though  furnish- 
ing little  or  no  nourishment,  helps  to  grind  up  organic 
materials,  just  as  the  gravel  does  in  a  chicken's  gizzard. 

The  ccelom  of  the  earthworm  is  not  continuous,  but  is  cut 
off  at  the  end  of  each  segment  by  a  membrane,  or  septum 
(Fig.  16).  The  watery  fluid  in  the  body-cavity  therefore 
cannot  pass  freely  from  one  end  of  the  body  to  the  other  as 


206  GENERAL  ZOOLOGY 

in  Ascaris.  This  is  of  small  consequence,  however,  for 
Lumbricus  has  a  well-developed  circulatory  system  of  the 
closed*  type,  with  arteries,  veins,  capillaries,  and  con- 
tractile vessels.  The  blood  is  forced  forward  by  con- 
tractions in  the  walls  of  the  large  dorsal  vessel,  flows 
through  the  arteries  to  capillaries  among  the  internal  or- 
gans and  in  the  skin,  then  returns  to  the  dorsal  vessel 
through  veins.  The  blood  acquires  nourishing  substances 
from  the  digestive  system  and  is  aerated  in  the  skin  capillar- 
ies. Like  that  of  vertebrates,  it  carries  both  nourishment 
and  gases.  Respiration  is  carried  on  wholly  through  the 
skin,  waste  gases  being  eliminated  from  the  blood  and  re- 
placed by  oxygen.  The  earthworm's  blood  contains 
haemoglobin,  and  is  hence  an  admirable  carrier  of  carbon 
dioxide  or  oxygen. 

An  earthworm  is  well  provided  with  excretory  organs,  for 
nearly  every  one  of  the  hundred  or  more  metameres  con- 
tains a  pair  of  nephridia.  These  are  slender  tubes  which 
coil  about  in  the  ccelom,  and  are  therefore  well  situated  to 
absorb  and  eliminate  liquid  waste  products.  A  nephridium 
begins  with  a  closed  funnel-like  expansion  in  one  metamere, 
passes  back  through  a  septum  into  the  next,  coils  back  and 
forth  three  times,  and  opens  to  the  exterior  through  a  little 
pore  in  the  body  wall. 

Self -protection. — Lumbricus  is  much  sought  for  food  by 
robins,  shrews,  and  other  animals.  It  is  perfectly  helpless 
when  captured,  but  escapes  from  many  of  its  enemies  by 
remaining  underground  except  at  night.  When  some  ani- 
mal tries  to  pull  it  from  the  burrow,  all  the  setae  are  pro- 
truded and  cling  with  great  tenacity.  If  the  body  breaks 
in  two  under  the  strain  no  particular  harm  results.  To  be 
sure,  the  portion  remaining  with  the  enemy  is  speedily  de- 
voured and  digested,  but  that  in  the  burrow  soon  grows  the 
parts  needed  to  make  a  complete  body  and  is  "as  good  as 

*  In  a  closed  circulatory  system  the  blood  is  always  within  the  vessels 
and  passes  through  a  regular  course;  in  an  open  system  it  flows  in  part 
through  vessels,  but  also  enters  large  irregular  spaces,  or  sinuses. 


PHYLUM  ANNELIDA  207 

new. "  There  is  a  limit,  however,  to  the  ability  of  fragments 
to  grow  into  complete  worms.  If  a  piece  containing  only  a 
few  metameres  is  cut  from  the  anterior  or  posterior  end,  a 
double-headed  or  double-tailed  monster,  which  is  incapable 
of  sustaining  itself,  is  produced.  Professor  T.  H.  Morgan 
has  made  many  curious  monstrosities  by  grafting  parts  of 
earthworms  together — individuals  with  the  body  single  in 
front  and  double  behind,  long  composite  worms  made  by 
fusing  parts  of  two  or  three  individuals,  etc. 

Earthworms  escape  dry  seasons  and  the  cold  of  winter  by 
burrowing  deep  in  the  ground  and  collecting  together  in 
clumps  for  mutual  protection.  The  slime  which  is  secreted 
from  the  surface  of  the  body  forms  a  tough  coating  on  the 
inside  of  the  burrow  so  that  there  is  little -danger  of  ob- 
struction by  the  caving  in  of  dirt.  The  tunnel  is  thus 
always  open  for  a  quick  retreat  if  the  necessity  arises. 

Race  Preservation. — Earthworms  are  hemaphroditic  but 
not  self-fertilizing.  During  the  breeding  season  two  in 
dividuals  frequently  come  together  so  that  their  anterio 
ends  overlap,  and  a  slime  tube  is  secreted  about  th 
bodies  of  both  (Fig.  83,  B).  The  openings  which  discharge 
spermatozoa  in  each  worm  are  thus  brought  opposite  the 
little  cavities  for  receiving  sperm  in  the  other  individual. 
When  the  pair  separate  each  has  been  supplied  with  sperm 
by  the  other  (that  is,  four  little  pockets  in  the  skin  have 
been  filled  with  spermatozoa),  and  each  is  prepared  to  lay 
fertile  eggs. 

Every  mature  earthworm  has  a  smooth  swollen  region, 
the  clitellum,  near  the  anterior  end  of  the  body.  This  gives 
off  a  glandular  secretion  which  forms  cocoons  for  receiving 
eggs  and  protecting  them  during  development.  When  a 
worm  is  about  to  deposit  a  cocoon  the  clitellum  forms  an 
elastic,  membranous  band  which  is  slipped  off  over  the  an- 
terior end  of  the  body,  just  as  a  ring  slips  from  a  finger. 
As  the  band  passes  over  the  openings  of  the  female  repro- 
ductive ducts,  eggs  are  discharged  inside  it;  and  as  it  goes 
across  the  pockets  where  the  spermatozoa  are  stored  some 


208  GENERAL  ZOOLOGY 

of  the  male  reproductive  cells  are  also  set  free  within  the 
moving  band.  When  it  pasess  off  the  body,  the  ends  con- 
tract and  a  little  hollow  cocoon  is  formed.  The  eggs  are 
fertilized  inside  the  cocoon  and  undergo  the  usual  series  of 
developmental  stages  (Fig.  64).  The  first  young  worm  to 
hatch  usually  devours  his  younger  relatives,  so  that  as  a 
rule  only  one  worm  is  produced  from  each  cocoon.  A  "  fer- 
tilized "  earthworm  does  not  use  all  its  stored  sperm  at  once, 
but  may  produce  a  dozen  or  more  cocoons  after  each  mating. 

GENERAL  REMARKS  ON  EARTHWORMS  AND  OTHER 
CH^ETOPODS 

An  earthworm's  body  is  made  up  of  a  series  of  very  simi- 
lar segments  and  more  than  half  of  these  may  be  lost  without 
destroying  the  ultimate  effectiveness  of  the  bodily  machine. 
If  the  control  of  the  earthworm's  functions  was  strongly 
ocalized  at  the  anterior  end,  as  in  some  bilaterally  symmet- 
rical animals,  such  a  loss  could  not  be  sustained.  In  this 
particular,  then,  the  worm  presents  a  primitive  condition  of 
metamerism,  which  is  said  to  be  homonomous,  because  the 
body  segments  are  much  alike. 

An  earthworm's  nervous  system  is  not  strongly  central- 
ized but  is  made  up  of  a  series  of  short  overlapping  units. 
It  consists  of  a  dorsal  ganglion  near  the  anterior  end,  two 
nerve  trunks  around  the  esophagus,  and  a  ventral  chain  of 
metameric  ganglia  throughout  the  rest  of  the  body.  As  a 
worm  crawls  along  the  impulse  for  contraction  originates  at 
one  end  and  is  passed  slowly  down  the  whole  length.  For 
example,  if  the  circular  muscles  at  the  anterior  end  contract, 
the  same  set  of  muscles  is  progressively  stimulated,  and  a 
wave  of  contraction  passes  backward.  If  the  worm  is  cut 
in  two  and  then  stitched  together  with  a  thread  so  that 
there  is  a  space  of  several  inches  between  the  cut  surfaces, 
the  worm  moves  about  as  if  it  had  a  continuous  body;  the 
pull  exerted  by  the  forward  piece  stimulates  the  one  behind 
to  move  in  the  usual  way.  Such  behavior  shows  that  the 


PHYLUM  ANNELIDA  209 

crawling  movements  are  brought  about  by  "  chain  reflexes  " 
which  pass  down  the  body,  each  part  setting  off  the  one 
behind. 

The  earthworm  possesses  a  great  number  of  small  sense 
organs  which,  though  slightly  more  abundant  toward  the 
ends,  are  distributed  rather  uniformly  in  the  skin  through- 
out the  body.  It  is  able  to  taste  or  smell  with  equal  facility 
at  any  point  on  the  outer  surface  and  therefore  shows  little 
or  no  cephalization  of  the  chemical  sense.  An  earthworm 
has  no  eyes,  but  is  sensitive  to  light  over  its  whole  surface, 
and  in  this  respect  also  shows  a  diffuse  condition.  In  each 
segment  of  the  body,  sense  organs  are  less  abundant  in  the 
middle  and  more  numerous  toward  the  anterior  and  posterior 
ends.  When  the  body  is  contracted,  therefore,  the  ma- 
jority of  these  organs  are  folded  into  the  grooves  between 
the  segments,  and  cannot  be  stimulated.  It  is  possible  that 
the  earthworm  may  rest  by  contracting  the  body  to  cover 
most  of  the  sense  organs,  just  as  a  man  closes  his  eyes  when 
asleep. 

Though  the  earthworm  is  structurally  and  physiologically 
a  chain  of  short  units  which  work  with  more  or  less  in- 
dependence and  in  a  rather  mechanical  way,  it  is  able  to 
carry  on  some  activities  which  require  the  correlation  of  all 
parts  of  the  body.  Above  the  pharynx  is  a  ganglion,  or 
" brain,"  and  there  are  three  giant  nerve  fibers  which  run 
through  the  whole  nervous  system  from  end  to  end.  When 
the  entire  body  wiggles,  the  activities  of  the  short  nervous 
units  are  probably  controlled  and  coordinated  from  the 
brain  through  these  long  fibers.  There  is  also  one  type  of 
sensation  which  is  apparently  localized  at  the  anterior  end. 
If  an  earthworm  is  allowed  to  crawl  about  on  a  moist  surface 
of  limited  area,  it  withdraws  when  it  comes  to  dry  spots  at 
the  edge,  but  if  the  prostomium  (or  upper  lip)  is  removed 
such  reactions  cease.  The  ability  to  discriminate  between 
wet  and  dry  surfaces  is,  however,  the  only  sense  that  is 
known  to  be  thus  localized. 

Though  the  earthworm  and  its  close  relatives  are  rather 


210 


GENERAL  ZOOLOGY 


strictly  homonomous  in  their  metamerism,  there  are  many 
other  chsetopods  which  are  heteronomous.  Many  of  the 
marine  worms  have  a  distinct  head  which  may  bear  several 
eyes,  tentacles,  and  other  sense  organs  (Fig.  84,  B),  thus 
showing  cephalization.  Some  have  the  metameres  differ- 
entiated into  several  groups,  so  that  there  are  separate  body 
regions  with  more  or  less  distinctive  functions. 


FIQ.  84. — Marine  annelids  on  sea  bottom.  Some  burrow  through  the  mud, 
others  make  tubes  of  leathery  secretions,  sand  grains,  bits  of  seaweed  or  lime, 
which  are  buried  in  the  mud  or  attached  to  rocks,  piles,  and  seaweeds.  A, 
Hydroides  in  calcareous  tubes  on  a  rock;  B,  Nereis;  C,  Polynce;  D,  Spirorbis  on 
seaweed;  E,  Cistenides  in  sand  tube;  F,  Chcetopterus  in  leathery  tube;  G,  Arenicola. 

Chsetopods  are  divided  mio  two  great  groups.  The 
Oligochceta  (few  bristled)  are  mostly  confined  to  land  and 
fresh  water,  the  earthwrorms  being  common  examples.  The 
small  aquatic  oligochsetes  have  rather  long  setae  and  are 
often  beautifully  transparent.  Under  the  microscope  all 
their  internal  organs  may  be  seen  through  the  body  wall. 
The  Polychceia  (many  bristled)  are  mostly  marine  and  show 
great  variation  in  structure  and  habits.  Many  species  live 


PHYLUM  ANNELIDA 


211 


in  tubes  which  they  make  from  secretions  and  bits  of  foreign 
substances,  such  as  sand  (Fig.  84,  E),  lime  (A,  D),  and  mucous 
secretions  (F).  Many  of  the  tube-building  species  have 
gills  on  the  head,  and  when  these  are  brightly  colored  they 
often  make  tne  bottom  of  the  ocean  look  like  a  flower  garden. 
Some  polychsetes  have  the  dorsal  surface  covered  with 
spines  or  protective  plates  (C). 

CLASS  2.     HIRUDINEA 

Leeches  (Fig.  85),  are  found  in  fresh  water,  on  land,  and  to 
some  extent  in  the  ocean.     They  are  without  setae  but  have 


A  B  C  o         E 

FIG.  85. — Leeches.  A,  attached  to  a  turtle's  leg;  B,  swimming;  C,  hiding 
beneath  a  rock  and  protruding  proboscis  to  catch  an  insect  larva;  D,  an  adult 
carrying  young  attached  to  its  ventral  surface;  E,  egg  cocoons. 

a  sucker  at  either  end.  The  rings  on  the  outside  of  the 
body  are  much  more  numerous  (70  to  100)  than  the  body 
metameres  (33) .  On  account  of  the  thickening  and  distor- 
tion of  the  septa,  the  metamerism  within  the  body  is  ob- 
scured; in  fact,  is  not  apparent  except  in  the  excretory  and 
nervous  systems.  The  ccelom  is  small  when  compared 
with  that  of  chsetopods. 

The  anterior  sucker  of  a  leech  contains  the  mouth,  but 
the  one  at  the  posterior  end  is  merely  an  adhesive  disc,  and 
the  anus  opens  in  front  of  it  on  the  dorsal  side-  Leeches 


212  GENERAL  ZOOLOGY 

vary  greatly  in  their  food  habits.  Some  live  largely  on 
insect  larvae  (Fig.  85,  C),  others  are  scavengers,  bub  the 
most  important  are  blood  suckers.  Some  of  the  blood 
suckers  puncture  the  skin  with  a  muscular  proboscis,  others 
bite  a  hole  with  three  small  teeth.  A  substance  is  secreted 
which  inhibits  the  coagulation  of  the  blood,  so  that  there 
may  be  no  interference  with  feeding.  The  alimentary  canal 
is  large  and  has  several  side  pockets.  When  an  opportunity 
occurs,  a  leech  gorges  itself  and  is  then  able  to  go  for  several 
months  without  food. 

The  breeding  habits  of  leeches  are  of  particular  interest. 
In  some  species  one  individual  sticks  a  packet  of  sperm  on 
the  outside  of  another,  and  the  spermatozoa  bore  in  through 
the  skin.  This  unique  method  of  fertilization  is  known  as 
hypodermic  impregnation.  Many  leeches  produce  cocoons 
Jike  those  of  earthworms  which  are  stuck  on  the  underside 
of  rocks  (Fig.  85,  E);  others  carry  the  eggs  and  young 
fastened  to  the  ventral  side  of  the  body  (D). 


CHAPTER  XX 
PHYLUM  MOLLUSCA 

Molluscs  are  bilaterally  symmetrical,  unsegmented  ani- 
mals which  usually  have  a  ventral  muscular  foot  and 
commonly  possess  a  calcareous  shell.  All  have  a  mantle 
of  some  sort.  This  is  a  covering  which  may  have  various 
uses,  such  as  protecting  the  body,  forming  the  shell,  and 
swimming.  Between  the  mantle  and  the  body  proper  there 
is  a  space,  the  mantle  cavity,  which  is  of  great  importance  in 
the  activities  of  molluscs.  There  are  five  molluscan  classes, 
but  only  three  are  of  general  importance. 

Class  1.  Gastropoda;  snails,  slugs,  nudibranchs;  with  a  large  flat 
creeping  foot,  and  usually  having  a  spirally  coiled  shell. 

Class  2.  Lamellibranchiata:  clams,  oysters,  ship  worms;  with  an 
axe-shaped  foot  and  a  shell  composed  of  two  flat  valves. 

Class  3.  Cephalopoda:  squids,  cuttle-fishes,  devil-fishes,  octopi; 
with  the  body  elongated  and  the  foot  divided  into  a  number  of  arms, 
which  usually  bear  suckers;  shell  usually  small  and  inclosed  within  the 
mantle,  or  absent. 

CLASS   1.     GASTROPODA 
THE  POND  SNAIL,  Physa  gyrina  Say 

Self -maintenance. — Physa  (Fig.  86)  lives  among  aquatic 
plants  where  there  is  an  abundance  of  food.  It  moves 
about  by  making  muscular  movements  of  its  broad  ventral 
foot.  If  a  Physa  is  observed  from  beneath  while  crawling 
in  a  glass  dish,  little  waves  will  be  seen  to  follow  each  other 
from  one  end  of  the  body  to  the  other.  These  indicate 
bands  across  the  body  where  the  foot  is  temporarily  pulled 
away  from  the  glass  and  moved  forward  a  little.  This  is  a 
slow  method  of  progress,  but  sure.  As  the  snail  moves 
along  a  slimy  trail  is  left  behind,  and  the  slime  is  of  consid- 

213 


214 


GENERAL  ZOOLOGY 


erable  importance  in  locomotion;  in  fact,  movement  is  not 
possible  without  it. 

Physa  is  able  to  vary  its  density  to  some  extent  by  pulling 
the  body  forward  or  backward  to  increase  or  decrease  the 
air  space  at  the  apex  of  the  shell  and  can  therefore  rise  to  the 
surface  of  the  water  or  fall  to  the  bottom  without  being  in 
contact  with  any  solid  object.  This  permits  it  to  spin 
threads  of  slime  in  all  directions  between  the  aquatic  plants 
(Fig.  86).  The  slime  threads  are  sticky  and  act  as  traps, 


A  B  C         0      E        F 

Fio.  86. — Activities  of  the  snail,  Physa.  A,  eggs  in  jelly  attached  to  a  plant 
stem;  B,  dropping  from  surface;  C,  taking  a  breath  through  the  surface  film; 
D,  resting;  E,  crawling  on  slime  thread;  F,  crawling  on  bottom. 

catching  minute  plants  and  animals.  After  they  have  been 
in  the  water  for  a  time,  Physa  eats  them  to  secure  the  ac- 
cumulated food.  They  are  also  made  use  of  as  roadways  in 
passing  from  one  part  of  a  pond  to  another. 

Physa  is  largely  vegetarian  in  its  food  habits.  Aside 
from  the  minute  particles  captured  by  the  slime  threads,  it 
gnaws  away  parts  of  plants  with  its  rasp-like  tongue,  and 
eats  the  scum  off  the  surface  of  the  water.  The  tongue,  or 
radula,  of  a  snail  is  a  very  complicated  mechanism,  with  a 


PHYLUM  MOLLUSCA  215 

great  number  of  little  teeth  arranged  in  regular  rows  and  an 
elaborate  system  of  muscles. 

A  snail's  digestive  system  is  of  the  tubular  type.  Food  is 
mixed  with  the  secretion  from  salivary  glands  in  the  mouth, 
passes  down  an  esophagus  into  a  crop,  where  it  may  rest  for 
a  time ;  then  enters  the  stomach  and  is  mixed  with  glandular 
secretions.  A  curved  intestine  leads  from  the  stomach  to 
the  anus,  which,  instead  of  being  at  the  posterior  end,  is 
situated  in  the  edge  of  the  mantle,  high  on  the  left  side  of 
the  body.  The  peculiar  position  of  the  anus  has  been 
brought  about  by  the  twisting  of  the  snail's  body  in  order 
to  become  adjusted  to  the  shell.  The  spiral  coil  of  the 
shell  makes  it  very  economical  of  space,  but  has  twisted  the 
soft  parts  of  the  body  to  such  a  degree  that  some  of  these  on 
the  left  side  have  become  rudimentary  or  absent. 

Physa  has  a  closed  circulatory  system,  with  a  heart, 
arteries,  veins,  and  capillaries  which  distribute  the  colorless 
blood  to  all  parts  of  the  body.  Blood-vessels  are  spread  out 
abundantly  in  the  walls  of  the  mantle  cavity,  and  it  is  there 
that  respiration  takes  place.  Physa  is  a  pulmonate  gas- 
tropod (a  snail  which  breathes  air)  and  the  mantle  cavity 
serves  as  a  lung.  It  comes  to  the  top  of  the  water  at  inter- 
vals, protrudes  its  breathing  tube  through  the  surface  film 
(C),  takes  a  deep  breath  of  air,  and  is  then  able  to  remain 
submerged  for  some  time.  On  account  of  the  twisting  of  the 
body  Physa  has  only  one  kidney,  the  right.  It  consists  of 
a  glandular  portion,  which  is  close  to  the  pericardial  cham- 
ber, or  the  space  around  the  heart,  and  a  long  tube  which 
opens  to  the  exterior  near  the  anus. 

Self -protection. — Physa  is  protected  by  its  calcareous 
shell,  which  furnishes  an  ever  ready  retreat  in  time  of 
danger,  and  shields  it  from  the  cold  of  winter.  The  shell 
also  makes  it  possible  to  endure  drought,  and  Physa  some- 
times lives  for  a  time  in  the  mud  left  after  a  pond  has  dried 
up.  It  is  not  a  protection  from  all  dangers,  however,  and 
many  fish  (perch,  pumpkinseed,  rock  bass,  etc.)  commonly 
eat  snails  whole. 


216  GENERAL  ZOOLOGY 

In  addition  to  dangers  which  threaten  from  without,  Physa 
is  the  subject  of  attacks  by  insidious  enemies  which  live 
within  its  body.  Chief  among  these  are  the  trematodes, 
which  pass  their  larval  stages  in  snails  and  then  pass  out  to 
mature  in  other  animals  (page  179) .  Against  such  parasites 
Physa  probably  has  no  means  of  protection. 

Race  Preservation. — Snails  are  hermaphroditic  and  dur- 
ing mating  both  the  individuals  taking  part  are  usually 
fertilized.  Physa  lays  its  eggs  in  a  little  drop  of  jelly  (Fig. 
86,  A).  This  adheres  to  solid  objects  in  the  water,  protects 
the  eggs,  and  furnishes  nourishment  for  the  young  snails 
developing  in  it. 

The  development  of  fertilized  snail  eggs  is  very  similar  to 
that  of  other  metazoans  .(Fig.  64),  except  in  regard  to  the 
cleavage.  In  very  early  cleavage  stages  cells  of  different 
sizes  are  formed.  Instead  of  lying  one  above  the  other, 
they  become  spirally  twisted  (  =  spiral  cleavage).  By  the 
direction  of  the  twisting,  the  form  of  the  shell  which  is  to  be 
formed  is  indicated.  Snails  are  of  two  sorts:  right-  and 
left-handed,  according  to  -whether  the  shell  coils  like  a 
right-  or  a  left-handed  screw.  Physa  is  left-handed,  but 
most  of  the  other  common  snails  are  not,  and  this  fact  may 
usually  serve  for  identification.  It  is  of  particular  interest 
that  the  direction  of  coiling  of  the  shell  is  indicated  in  the 
very  earliest  cleavage  stages. 

Young  Physas  remain  within  the  jelly  for  a  time  after 
they  are  fully  formed;  then  break  out  and  are  at  once  able 
to  shift  for  themselves.  The  parent  takes  no  care  of  the 
eggs  after  they  have  been  laid. 

GENERAL  REMARKS  ON  GASTROPODS 

Taking  the  animal  kingdom  as  a  whole,  the  most  special- 
ized and  most  successful  animals  are  those  which  have  be- 
come able  in  their  racial  evolution  to  live  on  land.  Among 
the  few  groups  that  have  become  perfectly  adjusted  to 
terrestrial  life,  snails  have  an  interesting  place  because  they 


PHYLUM  MOLLUSCA  217 

show,  at  the  present  time,  all  stages  of  transition  from  ocean, 
to  fresh  water,  and  to  land.  Most  marine  snails  breathe 
directly  from  the  water  through  feather-like  gills,  and  have 
a  free  swimming  larva  (veliger).  The  fresh- water  species 
do  not  have  free  swimming  larvae  and,  though  many 
breathe  like  their  marine  relatives,  some,  like  Physa,  have 
rudimentary  gills  and  respire  air  through  the  walls  of  the 
mantle  cavity.  Succinea  is  a  common  swamp  snail  which 
is  amphibious.  It  breathes  air,  but  may  live  in  the  water 
or  on  land,  and  is  as  often  found  perched  on  the  top  of  a 
cat-tail  as  among  the  plants  under  water.  Land  snails  lay 
their  eggs  in  holes  in  the  ground  and  in  rotten  logs;  they  all 
breathe  air  into  the  mantle-cavity  and  are  without  gills. 

Several  of  the  large  marine  snails  are  of  considerable 
economic  importance  because  they  are  pests  to  oyster  and 
clam  fishermen.  They  bore  holes  in  the  shells  of  other 
molluscs  with  their  radulas  and  extract  the  soft  parts. 
The  slugs,  which  are  snails  with  very  rudimentary  shells, 
and  other  land  snails  often  do  damage  to  gardens  by  gnaw- 
ing holes  in  the  vegetables.  The  large  snail  in  Europe, 
Helix  pomatia,  is  a  source  of  trouble  in  vineyards  and  fields, 
but  is  also  of  value  as  a  food.  It  is  raised  in  " snail  farms"* 
and  gathered  in  the  fields  for  local  consumption  and  export. 
Most  of  the  edible  snails  used  in  the  United  States  are  sold 
to  the  restaurants  in  the  large  cities.  The  Department  of 
Agriculture  has  recently  issued  a  bulletin  describing  meth- 
ods of  culture  and  suggesting  that  snail  farming  be  under- 
taken in  this  country. 

CLASS  2.    LAMELLIBRANCHIATA 

Here  belong  the  clams,  mussels,  oysters,  scallops,  ship 
worms,  and  other  "  bivalves. "  These  animals  obtain  their 
food  by  siphoning  large  quantities  of  water  through  the 
mantle  cavity  and  retaining  the  minute  particles.  Cur- 
rents created  by  the  cilia  covering  the  body  and  mantle 
bring  in  small  plants  and  animals  which  are  swept  into  the 


218 


GENERAL  ZOOLOGY 


mouth.  The  whole  alimentary  canal  is  ciliated  and  the 
muscles  accordingly  take  little  part  in  moving  the  food 
through  it,  which  is  very  unusual. 

Most  lamellibranchs  move  very  slowly  by  protruding  the 
foot  (Fig.  87  A',/)  and  pressing  against  the  bottom,  but  some 


D' 


FIG.  87.  —  Life  history  of  fresh  water  mussel.  A,  adult  clam  giving  off  glochidia 
through  excurrent  siphon;  A',  anatomy  of  mature  clam;  a,  adductor  muscle; 
/,  foot;  p,  marsupium,  or  swollen  portion  of  gill,  in  which  glochidia  mature  from 
eggs.  B,  B',  young  glochidia  on  bottom;  C,  C',  glochidia  encysted  in  fishes' 
fins;  D,  D',  young  clam. 

clams  are  able  to  leap  a  short  distance;  the  scallops  swim 
actively  by  squirting  water  from  the  mantle  cavity,  and  the 
fresh-water  Sphceridce  climb  about  on  aquatic  plants.  A 
number  of  clams  are  able  to  burrow  with  great  facility. 
The  oyster  moves  about  only  while  young,  for  it  grows 
fast  to  the  bottom  and  is  then  unable  to  change  its  position. 
On  this  account  oysters  are  often  smothered  by  mud 


PHYLUM  MOLLUSCA  219 

deposited  after  storms  or  floods.  The  ship  worm,  Teredo 
navaliSj  bores  in  piles,  wharves,  and  even  in  wooden  ships; 
hence  is  very  destructive  to  timber.  It  is  soft  and  worm- 
like,  the  shell  being  restricted  to  one  end  of  the  body  and 
serving  as  an  auger  for  boring. 

The  breeding  habits  of  lamellibranchs  are  quite  variable. 
Most  of  them  are  dioecious  and  some  species  show  sexual 
dimorphism,  the  females  usually  being  larger  than  the  males. 
Marine  species  usually  have  a  free-swimming  larva  (veliger) 
which  is  trochophore-like  (page  190),  but  those  in  freshwater 
generally  bring  forth  living  young.  The  life  history  of  the 
fresh-water  mussel  (Fig.  87)  shows  great  specialization 
and  is  well-adjusted  to  conditions  in  rivers,  ponds,  and  lakes. 
The  eggs  are  fertilized  within  the  mantle  cavity  of  the 
female;  then  pass  into  the  gills,  which  serve  as  brood- 
pouches,  or  marsupia,  and  become  greatly  swollen.  The 
larvae  developed  within  the  marsupia  are  known  as  glo- 
chidia,  and  there  are  two  types,  hooked  (Fig.  87,  B')  and 
hookless.  Both  are  usually  set  free  in  the  water  and  lie 
on  the  bottom  until  a  fish  comes  into  contact  with  them. 
A  hooked  glochidium  becomes  attached  to  the  fish's 
fins  (C)  and  grows  for  a  time  as  a  parasite  within  a  cyst. 
The  hookless  glochidia  live  as  parasites  on  the  gills  of 
fishes.  Both  types  mature  within  cysts,  then  break  out, 
ready  to  take  up  the  usual  activities  of  young  mussels  (D). 

Lamellibranchs  are  of  great  economic  value  in  the  United 
States.  Along  the  seacoast  oysters,  clams,  mussels,  and 
scallops  are  collected  for  food.  The  national  and  state 
governments  maintain  laboratories  for  the  propagation 
and  study  of  these  important  natural  resources.  The  ship 
worm,  on  the  other  hand,  causes  great  losses  and  the  govern- 
ment is  trying  to  find  better  means  to  control  or  exter- 
minate it.  Pearls  are  formed  when  small  foreign  bodies  or 
parasites  lodge  in  the  mantle  of  a  clam  and  become  covered 
with  layers  of  lime.  In  the  United  States  the  chief  source 
of  these  jewels  is  from  the  fresh- water  mussels  which  are 
dredged  from  the  bottoms  of  rivers  and  lakes.  The  shells 


220 


GENERAL  ZOOLOGY 


of  lamellibranchs  are  also  the  source  of  pearl  buttons,  and 
along  the  Mississippi  River  a  large  number  of  persons 
(3592  in  1915)  are  engaged  in  collecting  mussels  for  this 
industry.  A  recent  publication  of  the  Bureau  of  Fisheries 
lists  forty  species  which  are  of  value. 


CLASS  3.     CEPHALOPODA 

Most  cephalopods  (Fig.  88)  are  far  too  active  to  carry  a 
heavy  exoskeleton  about,  and  the  shell  is  as  a  rule  absent  or 


ABC 

FIG.  88. — Cephalopoda.     A,  an  octopus  or  devil-fish;  B,  squids  swimming  with 
fish  and  resting  on  bottom;  m,  mantle;  s,  siphon;  C,  squid  eggs. 

small  and  enclosed  within  the  mantle.     Aside  from  the  rare 
nautaloids,  there  are  two  common  types  of  cephalopods: 

(1)  squids,  are  elongated,  have  ten  arms,  and  swim  actively; 

(2)  devil-fishes,  or  octopods,  have  eight  arms,  climb  more 
than  they  swim,  and  have  short  bodies.     The  mantle  (ra)  in 
all  cephalopods  forms  a  muscular  tube  which  encloses  the 
whole  body,  except  the  head  and  arms.     The  method  of 
swimming  is  peculiar.     Water  is  drawn  into  the  mantle 


PHYLUM  MOLLUSCA  221 

cavity  and  forced  out  of  the  siphon  (s)  so  powerfully  that 
the  whole  animal  is  propelled  through  the  water  backward. 
This  movement  is  accelerated  and  guided  by  the  lateral 
fins,  thus  making  cephalopods  expert  swimmers. 

The  food  of  these  molluscs  consists  of  small  fish  and  other 
marine  animals.  After  being  captured,  the  prey  is  held 
with  the  suckers  on  the  arms,  torn  apart  with  the  parrot- 
like  beak,  and  ground  up  somewhat  by  the  radula  before 
passing  down  to  the  stomach.  On  account  of  the  contrac- 
tions of  the  mantle  while  swimming,  all  the  internal  organs 
are  subjected  to  great  pressure  and  food  would  be  regurgi- 
tated through  the  mouth  if  special  valves  were  not  present. 
Valves  are  also  present  in  the  blood-vessels  to  prevent  back- 
flow,  and  there  are  two  accessory  hearts  which  pump 
blood  through  the  plume-like  gills. 

All  cephalopods  have  large  eyes  on  the  sides  of  the  head. 
These  resemble  those  of  vertebrates  in  their  general  plan, 
but  differ  markedly  in  details  of  structure  and  in  develop- 
ment. The  eyes  enable  squids  and  octopi  to  see  with  great 
acuity,  and  the  pursuit  of  active  animals  for  food  is  there- 
fore easy.  When  a  squid  or  an  octopus  is  set  upon  by  an 
enemy,  a  great  cloud  of  ink  is  squirted  into  the  water  through 
the  anus.  Its  movements  are  thus  obscured,  and  escape 
is  made  possible. 

The  breeding  habits  of  cephalopods  are  very  peculiar. 
A  male  takes  a  spermatophore,  or  packet  of  sperm,  on  one 
of  his  arms  and  places  it  within  the  mantle  cavity  of  the 
female.  When  a  female  lays  her  eggs  she  ruptures  the  sper- 
matophore, a  great  cloud  of  spermatozoa  swarm  over  them, 
and  fertilization  is  thus  insured.  In  some  species  the  male 
breaks  off  the  tip  of  his  arm  and  leaves  it  in  the  female 
with  the  spermatophore  enclosed.  When  this  arm  tip  was 
first  discovered  by  one  of  the  older  zoologists,  it  was  de- 
scribed as  a  parasitic  worm  which  was  believed  to  live 
within  the  female.  A  female  squid  or  octopus  deposits  her 
eggs  on  the  bottom  of  the  ocean  or  attaches  them  to  a 


222  GENERAL  ZOOLOGY 

sea- weed  (C).     She  then  leaves  them  to  develop  without 
further  care. 

Squids  are  commonly  eaten  for  food  in  Italy  and  are  com- 
ing to  be  used  in  other  countries.  They  are  also  much  sought 
for  bait  by  fishermen.  Though  some  squids  attain  a  total 
length  of  forty  feet,  they  are  as  a  rule  very  shy,  and  their 
reputed  attacks  on  men  are  probably  fictitious. 

GENERAL  REMARKS  ON  MOLLUSCS 

The  Mollusca  are  remarkable  among  other  phyla  of 
bilaterally  symmetrical  animals  because  they  are  complex 
in  structure  but  strictly  non-met americ.  Though  the  tro- 
chophore-like  larva  (page  190)  indicates  possible  affinities 
with  the  Annelida,  Echinodermata,  Rotifera,  etc.,  the 
structure  of  adult  molluscs  is  wholly  unlike  that  of  other 
animals.  The  Mollusca  therefore  have  no  close  relation- 
ship with  any  other  phylum,  and  have  developed  along 
original  lines. 

The  most  striking  characteristics  of  the  phylum  as  a  whole 
are  the  foot,  the  shell,  the  mantle,  and  the  simple  nervous 
system,  but  all  these  structures  differ  greatly  in  different 
classes.  The  foot  is  flat  in  gastropods,  hatchet-shaped  in 
lamellibranchs,  and  divided  into  eight,  ten,  or  more  arms  in 
cephalopods.  The  shell  is  univalve  in  gastropods,  bivalve 
in  lamellibranchs,  and  usually  rudimentary  or  absent  in 
cephalopods.'  Squids  and  devil-fishes  have  a  sac-like 
mantle  which  encloses  most  of  the  body ;  clams  usually  have 
the  mantle  divided  into  two  great  lobes,  completely  cover- 
ing the  soft  body;  but  in  snails  the  greater  part  of  the  body 
may  be  extended  outside  the  mantle  cavity. 

Except  in  cephalopods  the  nervous  system  is  as  a  rule 
poorly  developed  and  its  structure  is  rather  similar  in  all  the 
classes.  It  consists  of  three  or  four  pairs  of  ganglia  and  a 
few  connective  nerves.  Molluscs  show  in  a  striking  way 
that  the  degree  of  specialization  in  nervous  structures  is 
associated  with  activity.  Most  of  the  slow,  sluggish  clams 


PHYLUM  MOLLUSCA  223 

are  practically  without  special  sense  organs  (except  those 
for  balancing  the  body  and  for  testing  the  surrounding 
water),  and  depend  largely  on  reflexes  in  their  daily  activi- 
ties, but  the  scallops  (Pecten)  which  swim  actively  have 
a  double  row  of  eyes  around  the  edges  of  the  mantle.  The 
snails  are  somewhat  more  active  than  clams,  and  have 
simple  eyes,  organs  of  taste,  and  other  sensory  structures, 
but  are  at  best  rather  slow.  The  most  active  molluscs  are 
the  cephalopods,  and  they  show  the  most  elaborate  sen- 
sory organs:  well-developed  eyes,  balancing  organs,  taste 
organs,  etc. 

The  mental  powers  of  all  molluscs  are  very  limited.  As 
would  be  expected  from  their  superior  sensory  equipment, 
the  cephalopods  excel  other  members  of  the  group.  They 
are  able  to  see  and  pursue  their  prey,  elude  their  enemies, 
and  carry  on  other  activities  which  are  impossible  for  the 
sluggish  clams  and  snails.  One  investigator  taught  a  squid 
to  take  food  from  a  hole  in  a  glass  tube,  and  the  food  was 
extracted  more  quickly  after  some  experience  had  been 
gained. 


CHAPTER  XXI 

i 

PHYLUM  CHORDATA 

The  phylum  Chordata  includes  all  the  animals  commonly 
known  as  vertebrates  and  a  number  of  others  that  are 
somewhat  less  familiar.  All  chordates  have  a  thick  rod 
running  through  the  body  which  is  known  variously  as  the 
chorda  dorsalis,  chorda,  or  notochord.  This  is  much  like 
a  straight  Bologna  sausage  in  form  and  consistency,  having 
a  membrane  over  the  outside  and  a  filling  of  pulp  within. 


integument-  , 

muscles  of 
"body  H/ar//^**, 

chorda-  ,,.„ 

nervous  ^ 
system 
-enferon-^ 
.  blood  . 


-kidney  — 

orary 
—  coelom 


Fio.  89. — Cross  sections  of  an  earthworm  and  a  lamprey-eel  compared.     Note 
the  difference  in  the  relative  position  of  the  central  nervous  system. 

The  chorda  is  strictly  a  skeletal  structure,  and  has  nothing 
to  do  with  the  spinal  cord,  which  is  a  part  of  the  nervous 
system. 

Besides  the  chorda,  the  chordates  have  two  other  dis- 
tinctive features:  (1)  the  gill  slits  and  the  gill  arches;  and 
(2)  a  tubular  nervous  system  situated  dorsal  to  the  ali- 
mentary canal  (Fig.  89) .  The  gill  slits  are  paired  openings 
from  the  pharynx  to  the  outside  of  the  body.  In  most 
aquatic  chordates  they  serve  as  exits  for  the  water  drawn 

224 


PHYLUM  CHORDATA  225 

through  the  mouth  and  used  for  respiration,  but  are  present 
in  terrestrial  forms  only  during  development.  Between 
the  slits  run  the  aortic  arches  carrying  blood,  and  these 
great  arteries  are  often  supported  by  cartilaginous  or  bony 
structures.  The  nervous  system  of  chordates  is  formed 
during  development  by  the  turning  in  of  the  ectoderm  on 
the  dorsal  surface  (Fig.  92).  At  first  there  is  simply  a 
longitudinal  depression  (A),  and  the  deeper  part  of  this  is 
later  pinched  off  (B,  C)  to  form  a  longitudinal  tube  inside 
the  body  wall  (C).  This  tube  usually  becomes  thickened 
and  expanded  somewhat  at  the  anterior  end  to  form  a  brain, 
and  nerves  grow  out  from  it  to  all  parts  of  the  body,  but 
even  in  ah  adult  man  it  is  still  tubular  in  its  fundamental 
structure. 

In  some  of  the  most  primitive  living  chordates,  such  as 
the  lamprey  eels  and  the  lancelets,  the  chorda  is  functional 
and  constitutes  practically  the  sole  skeletal  structure  within 
the  body,  but  in  the  great  majority  of  the  vertebrates  it  is 
replaced  to  a  greater  or  lesser  extent^  by  a  series  of  verte- 
brae which  together  make  up  the  spinal  column,  or  "  back- 
bone." There  is  also  a  transition  in  the  aortic  arches  in 
passing  from  primitive  to  more  specialized  chordates. 
The  aquatic  representatives  have  functional  gill  slits  and 
well-developed  aortic  and  skeletal  arches.  The  frogs  and 
toads  begin  active  life  with  a  fish-like  form  and  functional 
gills,  but  later  lose  the  gills  and  have  the  blood  diverted 
to  the  lungs  and  skin  for  aeration.  Reptiles,  birds,  and 
mammals  never  breathe  through  gills,  but  nevertheless  have 
gill  slits  and  gill  arches  during  early  developmental  stages. 
The  primitive  arterial  arches  degenerate  in  the  adults  or 
are  diverted  to  supply  new  parts  with  blood  (Fig.  105). 
The  skeletal  parts  of  the  arches  which  serve  to  support  gills 
in  primitive  chordates  are  for  the  most  part  transformed 
into  jaws,  throat  cartilages,  and  the  bones  of  the  face  in 
more  specialized  types. 

Thus,  though  the  same  fundamental  plan  of  structure  is 
present  in  all  chordates,  parts  may  be  so  modified  as  to 


226  GENERAL  ZOOLOGY 

Completely  change  their  form,  and  have  often  during  evolu- 
tion come  to  serve  entirely  different  functions  from  those 
for  which  they  were  originally  used.  Such  changes  have 
usually  gone  with  modifications  in  modes  of  life.  Though 
all  the  primitive  chordates  are  largely  aquatic,  the  special- 
ized forms  are  nearly  all  terrestrial  and  hence  fitted  to 
breathe  air.  One  of  the  most  interesting  features  of  the 
past  evolution  of  the  vertebrates  has  been  their  migration 
from  water  to  land. 

The  phylum  Chordata  is  divided  into  four  subphyla. 

Subphylum  1.  Enteropneusta. — This  group  includes  cer- 
tain worm-like  animals.  Dolichoglossus  lives  buried  in  the 
mud  along  the  seashore  (Fig.  90,  /).  Its  chorda  is  small 
and  contained  within  a  swollen  proboscis  which  projects 
above  the  mouth.  The  body  is  divided  into  three  regions 
—proboscis,  collar,  and  trunk.  A  large  number  of  paired 
gill  slits  connect  the  pharynx  with  the  exterior.  Dolicho- 
tilossus  extracts  its  food  from  mud  which  is  forced  into  the 
mouth  at  the  front  of  the  collar,  passes  through  the  straight 
enteron,  and  leaves  the  body  through  the  anus  at  the  poste- 
rior end. 

The  larvae  of  some  of  the  Enteropneusta  are  tornarise, 
which  are  much  like  the  larvae  of  certain  echinoderms,  and 
resemble  a  trochophore  (page  190)  in  general  structure. 
This  has  led  some  zoologists  to  believe  that  chordates  arose 
from  a  remote  ancestor  which  resembled  a  trochophore,  and 
that  there  is  relationship  between  echinoderms,  annelids, 
molluscs'/ and  other  animals  originating  from  a  trochophore- 
like  larva. 

Subphylum  2.  Tunicata. — The  sea-squirts  (Fig.  90,  g,  h) 
are  degenerate  animals  which  grow  attached  to  piles  and 
other  objects  in  the  ocean.  They  are  covered  by  tough 
" tunics,"  through  which  there  are  two  openings  for  the 
entrance  and  exit  of  water.  When  taken  from  the  ocean 
two  little  streams  of  water  are  ejected  from  the  body; 
hence  the  common  name,  "  sea-squirt."  In  feeding,  water, 
is  strained  through  a  porous  pharynx,  which  catches  food 


PHYLUM  CHORDATA 


227 


particles  and  passes  them  on  to  the  stomach  and  intestine. 
The  anus  opens  near  the  exhalent  aperture  inside  the  tunic. 
Many  species  of  sea-squirts  are  colonial  and  form  masses 
known  to  fishermen  as  "sea  pork"  (&,  ti). 

Some  tunicates  are  not  attached  but  swim  at  the  surface 
of  the  ocean.     The  salpas,  for  example  (Fig.  90,  c,  d)  are 


FIG.  90. — Simple  Chordates.  The  sub-phyla  represented  are  as  follows: 
a-d,  g,  h,  Tunicata;  e,  Cephalochorda ;  /,  Enteropneusta.  The  separate  figures 
are:  a,  tunicate  tadpoles;  6,  Leptoclinum,  or  "sea  pork,"  a  colonial  tunicate; 
c,  a  chain  Salpa ;  d,  Salpa,  a  swimming  tunicate ;  e,  amphioxus ;  /,  Dolichoglossus ; 
g,  Molgula,  a  typical  "sea  squirt;"  h,  Amarcecium,  a  colonial  tunicate. 

beautifully  transparent,  barrel-shaped  animals ^which  swim 
by  drawing  water  into  the  front  of  the  tunic^and  forcing 
it  out  behind.  They  occur  singly  or  in  chain-like  colonies. 
The  Larvacea  are  another  group  of  free-swimming  tunicates 
which  resemble  minute  tadpoles  in  their  general  form. 

Most  adult  tunicates  have  the  chorda  greatly  reduced 
or  absent,  but  all  of  them  develop  from  little  tadpoles  in 


228  GENERAL  ZOOLOGY 

which  it  is  well-developed  and  functional.  The  Larvacea 
(a)  therefore  .represent  the  most  primitive  types.  Most  of 
the  common  sea-squirts  have,  like  barnacles  (Fig.  32), 
undergone  retrogressive  metamorphosis,  and  as  adults  are 
adapted  to  a  sessile  existence. 

Subphylurn.  3.  Cephalochorda. — The  lancelets  (Fig.  90,  e) 
are  small  fish-like  animals  which  bury  themselves  tail  first 
in  sandy  ocean  beaches, below  low-tide  mark.  Amphioxus 
is  a  well-known  representative  which  has  been  much  dis- 
cussed by  evolutionists  as  a  possible  ancestor  of  vertebrates. 
It  has  a  well-developed  chorda  and  numerous  paired  gill 
slits  opening  from  the  pharynx  to  the  exterior.  The  body 
wall  is  divided  into  a  large  number  of  muscle  segments,  or 
myomeres.  Along  the  whole  dorsal  margin  there  is  a  nar- 
row fin  which  is  continuous  posteriorly  with  a  broader 
caudal  fin.  On  the  ventral  side  of  the  body,  a  short  medial 
fold  (ventral  fin)  reaches  forward  from  the  caudal  fin,  and 
in  front  of  this  two  lateral  (metapleural)  folds  extend  to 
the  region  of  the  mouth.  The  median  fins  and  the  lateral 
folds  are  continuous,  but  have  the  same  arrangement  as 
the  separate  fins  of  true  fishes.  The  metapleural  folds 
have,  therefore,  been  considered  to  be  the  forerunners  of 
the  paired  limbs  of  the  vertebrates.  The  cephalochord- 
ates  excel  Enteropneusta  and  Tunicata  in  their  greater 
cephalization  and  show  closer  affinities  with  the  vertebrates. 

Subphylum  4.  Vertebrata. — This  group  includes  the  eel- 
like  cyclostomes,  sharks  and  skates,  salamanders,  frogs, 
reptiles,  birds,  and  mammals.  Most  vertebrates  have  two 
characteristics  which  usually  serve  to  separate  them  readily 
from  other  chordates:  (1)  vertebrae  replace  the  chorda, 
which  is  usually  present  as  such  only  during  embryonic 
development;  (2)  two  pairs  of  lateral  appendages,  or  limbs, 
are  present.  Though  vertebrates  as  adults  are  rather  di- 
verse in  structure  and  habits,  they  are  all  built  according 
to  the  same  fundamental  plan  and  pass  through  similar 
stages  in  development.  There  is,  therefore,  no  doubt  that 
vertebrates  are  genetically  related  ana,  Decause  their  evolu- 


PHYLUM  CHORDATA  229 

tion  has  been  more  recent  than  that  of  any  other  group  of 
animals,  more  facts  are  available  concerning  the  lines 
specialization  has  followed. 

In  the  most  primitive  vertebrates  the  chorda  is  functional 
throughout  life^nd  the  vertebrae  appear  only  as  little  carti- 
lages between  the  myomeres,  or  muscle  segments.  But  as 
the  complexity  of  the  skeleton  progresses  in  the  vertebrate 
series,  there  is  a  transition  from  chorda  to  cartilage,  and 
again  to  bone.  Some  vertebrates  are  as  adults  only  a  little 
beyond  the  chorda  stage/  others,  like  the  sharks,  have 
developed  a  cartilaginous  skeleton,  but  go  no  further.  The 
majority  of  vertebrates,  however,  pass  through  three  stages 
in  their  development:  (1)  having  a  chorda,  replaced  by 
(2)  cartilaginous  vertebrae,  which  in  turn  give  place  to  (3) 
bone.  Paired  limbs  %  like  wise  show  a  rather  definite  se- 
quence in  their  specialization.  They  are  absent  in  the 
most  primitive  vertebrates  and  appear  first  as  flaps  sup- 
ported by  cartilaginous  rays.  Fins  with  bony  rays  are  the 
next  step,  and  these  are  later  replaced  by  pentadactyl  (five- 
toed)  limbs.  The  pentadactyl  limb  represents  a  great  ad- 
vance in  racial  development  when  compared  with  a  fin, 
bi;t  it  has  now  been  so  greatly  modified  in  many  groups  as 
to  be  scarcely  recognizable.  A  bird's  wing  or  a  horse's 
foot  show  little  indication  of  the  ancestral  five-toed  condi- 
tion. Most  snakes  and  the  blind  worms  have  lost  the 
paired  appendages  completely. 

The  vertebrate  circulatory  system  also  affords  a  good 
illustration  of  progressive  specialization  and  development. 
The  most  primitive  vertebrates  have  a  tubular  heart  (Fig. 
105),  bent  somewhat  to  be  sure,  but  still  nothing  but  a 
tube,  which  pumps  blood  through  six  aortic  arches  to  the 
gills.  Very  soon  the  number  of  arches  is  reduced  to  four, 
then  to  three*  In  birds  and  mammals  the  arches  appear 
as  such  only  in  early  development,  and  in  the  adult  condi- 
tion all  but  a  single  arch  on  one  side  of  the  body  have  been 
lost  or  diverted  to  other  uses.  In  fishes  the  heart  presents 
a  simple  tubular  condition;  in  amphibians  and  in  most  rep- 


230  GENERAL  ZOOLOGY 

tiles  it  is  three-chambered  and  pumps  " half-pure"  blood. 
Birds  and  mammals  show  a  four-chambered  condition; 
the  blood  on  one  side  has  been  aerated  and  that  on  the 
other  is  charged  with  waste  products. 

There  are  three  kinds  of  kidneys  present  in  vertebrate 
animals.  These  are  believed  to  represent  parts  of  a  con- 
tinuous series  of  nephridia,  somewhat  like  those  of  annelids, 
but  in  no  vertebrate  is  more  than  one  of  the  three  functional 
at  a  time.  The  most  primitive  vertebrates  (Cyclostomes) 
have  only  one  kidney,  the  pronephros,  or  head-kidney. 
Fishes  and  amphibians  have  a  pronephros  during  early 
development,  but  it  soon  degenerates  and  is  replaced  by 
another  kidney,  the  mesonephros.  Reptiles,  birds,  and 
mammals  have  first  a  pronephros,  then  a  mesonephros, 
but  their  functional  kidney  during  adult  life  is  the  meta- 
nephros,  which  develops  after  the  other  two  have 
degenerated. 

Other  lines  of  vertebrate  specialization  might  be  described, 
but  enough  has  been  said  to  show  that  it  is  possible  to 
speak  in  more  positive  terms  concerning  this  group  of  ani- 
mals than  of  any  of  those  which  have  been  discussed  in 
the  preceding  pages.  Perhaps  the  most  interesting  thing 
to  keep  in  mind  as  we  pass  from  primitive  to  specialized 
vertebrates,  is  that  in  general  evolution  has  been  from 
aquatic  modes  of  existence  to  life  on  land.  Structures  and 
functions  have  been  accordingly  modified.  The  fin,  for 
example,  was  good  enough  as  an  organ  of  locomotion  in 
the  water,  but  cannot  compare  with  a  jointed  leg  ending 
in  toes  for  progress  on  land. 

There  are  seven  classes  of  vertebrates  which  may  be 
briefly  outlined  as  follows: 

Class  1.  Cyclostomata.  Eel-like  vertebrates  with  a  chorda  and 
median  fins,  but  without  paired  fins  or  true  jaws.  Lampreys  and 
hagfishes. 

Class  2.  Elasmobranchii.  Fish-like  vertebrates  with  cartilaginous 
skeletons,  jaws,  placoid  scales,  and  a  persistent  chorda.  Sharks, 
skates,  rays. 


PHYLUM  CHORDATA  231 

Class  3.  Pisces.  Fish-like  vertebrates  with  cartilaginous  or  bony 
skeletons,  true  jaws,  scales,  and  fins  supported  by  rays.  True  fishes. 

Class  4.  Amphibia.  Slimy-skinned  vertebrates  without  exoskele- 
tons;  with  pentadactyl  limbs;  no  claws  on  toes;  heart  three-chambered. 
The  young  are  aquatic  and  breathe  by  means  of  gills;  the  adults  are 
usually  more  or  less  terrestrial  and  breathe  through  lungs.  Frogs,  toads, 
salamanders. 

Class  5.  Reptilia.  Scaly-skinned  vertebrates,  which  breathe  by 
means  of  lungs  and  have  a  three-  or  four-chambered  heart;  claws  on 
the  toes.  Lizards,  snakes,  turtles,  crocodiles,  etc. 

Class  6.  Aves.  Feathered,  scaly-skinned  vertebrates  with  four- 
chambered  heart  and  warm  blood,  but  no  teeth.  Birds. 

Class  7.  Mammalia.  Warm-blooded  vertebrates  with  hair  and 
mammary,  or  milk,  glands.  Whales,  seals,  bats,  dogs,  horses,  monkeys, 
etc. 

CLASS  1.     CYCLOSTOMATA 

The  cyclostomes  lack  true  jaws  and  are  without  paired 
lateral  appendages.  The  nasal  pit  is  single  instead  of 
double  as  in  other  vertebrates,  and  there  are  from  seven  to 
fourteen  pairs  of  gill  slits.  Two  kinds  of  cyclostomes  are 
living  at  the  present  time — hagfishes  and  lampreys. 

The  hagfishes  are  found  only  on  muddy  bottoms  in  the 
ocean.  They  bore  their  way  into  fishes  in  order  to  eat  out 
the  soft  parts,  and  are  a  source  of  great  annoyance  to  fisher- 
men because  they  commonly  attack  shad,  cod,  and  other 
fish  caught  in  nets  or  on  hooks.  They  have  a  single  tooth 
which  they  use  for  boring  into  their  victims. 

Lampreys  live  both  in  the  ocean  and  in  fresh  water. 
They  have  a  cartilaginous  funnel  around  the  mouth  which 
is  used  as  a  sucking  disc  for  feeding  or  attachment.  There 
are  numerous  teeth  inside  the  mouth  and  on  the  tongue. 
Petromyzon  marinus  (Fig.  89,  B;  91,  ^4.)  is  a  common  species 
about  the  mouths  of  rivers  along  the  Atlantic  coast  of  the 
United  States.  It  lives  on  blood  which  is  obtained  by 
attaching  the  oral  disc  to  fishes  and  rasping  a  hole  with 
the  tongue.  The  larva  of  this  species  differs  markedly 
from  the  adult,  and  shows  several  characteristics  which 
resemble  those  of  Amphioxus  (Fig.  90,  e).  Another  cyclo- 


232 


GENERAL  ZOOLOGY 


, 
stome,  the  brook  lamprey  of  North  America,  Lampetra 

wilderi,  has  interesting  breeding  habits.  Stones  are  car- 
ried in  the  mouth^and  a  space  is  thus  cleared  in  which 
mating  take  splace.  This  species  does  not  eat  as  an  adult, 
and  is  therefore  not  harmful  to  fishes. 


FIG.  91. — Cyclostomes  and  clasmobranchs.     A,  lamprey-eels,  resting  and  at- 
tacking a  shark;  B,  shark;  C,  sting-ray. 


CLASS  2.     ELASMOBRANCHII 

Elasmobranchs  (Fig.  91,  B,  C)  resemble  true  fishes  in 
general  form  and  in  having  two  pairs  of  lateral  fins,  but 
differ  from  them  in  possessing  a  notochord  when  adult, 
placoid  scales  (Fig.  93),  a  spiral  valve  in  the  intestine,  clasp- 
ing organs  in  the  male,  and  in  lacking  an  air-bladder  and 
a  gill-cover,  or  operculum.  They  are  therefore  placed  in 
a  separate  class. 

There  are  two  common  types  of  elasmobranchs,  sharks 
and  rays.  The  sharks  and  dogfishes  are  cylindrical  or 


:T/LU: 


PHYLUM  CHORD AT A 


233 


compressed  and  swim  actively  about  seeking  for  food. 
They  are  voracious  feeders  on  fish  and  other  animals;  there- 
fore, are  readily  caught  on  a  hoojUL  Though  one  species 
attains  a  length  of  over  thirty  feet  and  sometimes  attacks 
man,  most  sharks  are  harmless,  except  for  their  ravages 
among  fishes  and  other  small  animals.  They  are  of  some 


p 

FIG.  92.  FIG.  93. 

FIG.  92. — Three  cross  sections  (A,  B,  C)  of  an  embryonic  vertebrate  to  show 
formation  of  nervous  system,  chorda,  and  coelom.  6,  membrane  surrounding 
ccelom;  c,  chorda;  e,  enteron;  n,  nerve  tube. 

FIG.  93. — Section  through  a  placoid  scale  of  an  elasmobranch.  d,  dentine; 
p,  "pulp;"  x,  enamel;  y,  integument. 

value  as  fertilizer^  and  for  the  oil  which  is  extracted  from 
their  livers.  The'  flesh  is  also  canned  to  some  extent  and 
sold  as  a  cheap  substitute  for  the  true  fishes.  Sharks  cause 
great  losses  by  destroying  lobsters  and  valuable  food  fishes. 
,  The  skates  and  rays  have  broad  flat  bodies  and  commonly 
lurk  on  the  bottom  of  the  ocean  awaiting  small  animals 


234  GENERAL  ZOOLOGY 

which  they  capture  and  devour.  The  sting  ray  (Fig,  91, 
C)  often  inflicts  painful  wounds  on  the  bodies  of  bathers 
and  fishermen.  It  has  a  barbed  poison  spine  at  the  base 
of  its  tail  which  is  driven  with  great  force  into  any  animal 
that  molests  it.  A  tropical  ray,  the  torpedo,  has  electrical 
organs  in  its  body  which  are  under  the  control  of  the  nerv- 
ous system.  These  are  capable  of  giving  powerful  shocks 
and  serve  as  effective  weapons  of  defense  or  offense. 

The  placoid  scales  (Fig.  93)  of  elasmobranchs  are  of  par- 
ticular interest  because  their  structure  closely  resembles 
that  of  the  teeth  in  the  mouths  of  true  fishes,  amphibians, 
reptiles,  and  mammals.  Each  is  covered  with  a  layer  of 
enamel,  which  surrounds  a  bony  dentine  layer,  and  the 
interior  is  filled  with  "pulp"  containing  blood  vessels, 
nerves,  and  connective  tissue.  It  is  believed  that  the  teeth 
of  vertebrates  have  been  derived  from  placoid  scales  such 
as  the  sharks  and  rays  possess.  In  elasmobranchs  these 
organs  occur  in  the  skin  over  the  whole  body  and  in  the 
mouth.  Those  in  the  mouth  are  on  a  portion  of  the  ecto- 
derm which  has  been  turned  inside;  they  are  particularly 
large  and  arranged  in  regular  rows.  In  other  vertebrates 
the  teeth  are  restricted  to  the  mouth,  or  in  other  words/ 
placoid  scales  are  lacking  from  the  skin. 


CHAPTER  XXII 
SUBPHYLUM  VERTEBRATA,  CLASS  3,  PISCES 

In  true  fishes  the  chorda  is  usually  replaced  by  bony 
vertebrae,  the  gills  are  protected  by  an  operculum,  or  "  gill- 
cover/'  and  the  body  is  covered  with  scales  which  are  never 
of  the  placoid  type,  as  in  sharks,  but  consist  of  little  bony 
plates  which  overlap  like  shingles.  In  general  form  and 
position^the  fins  are  much  like  those  of  sharks,  but  are  often 
supported  by  bony  rays.  There  are  two  subclasses  of 
fishes : 

Subclass  1.  Teleostomi.  Fishes  with  a  skeleton  composed  largely 
of  bone;  with  gills,  but  usually  without  lungs. 

Subclass  2.  Dipnoi.  Fishes  with  a  skeleton  made  up  of  cartilage 
and  bone,  and  with  a  single  or  paired  lung. 

As  an  example  of  the  subclass  Teleostomi,  the  yellow 
perch,  which  is  one  of  the  commonest  and  most  widely 
distributed  fishes  in  North  America,  will  be  considered. 

THE  YELLOW  PERCH,  Perca  Flavescens  Mitchill. 

Self-maintenance. — The  perch  (Fig.  94)  is  versatile  in 
every  way,  and  this  is  particularly  true  of  its  food  habits. 
It  may  feed  on  microscopic  animals  in  the  water  such  as 
Daphnia  or  Cyclops  (Fig.  31),  or  eat  snails,  insect  larvae, 
aquatic  insects,  minnows,  fish  eggs,  and  even  plants.  The 
minute  animals  are  secured  by  straining  the  water  which 
is  continually  entering  the  mouth  for  respiratory  purposes 
and  passing  out  behind  the  operculum.  This  straining  is 
accomplished  by  a  series  of  minute  finger-like  projections, 
the  gill-rakers,  attached  along  the  inner  margins  of  the 
gill-arches  and  spread  across  the  spaces  where  water  leaves 

235 


236 


GENERAL  ZOOLOGY 


the  body.  Small  particles  of  food  are  caught  and  held 
until  they  can  be  swallowed.  A  perch  may  swim  about 
in  a  swarm  of  microscopic  animals  (Fig.  94,  B)  and  strain 
out  enough  of  them  to  completely  fill  its  alimentary  canal. 
For  capturing  larger  prey,  such  as  minnows  and  insects, 
the  perch's  mouth  is  provided  with  sharp,  backwardly 


C  D 

FIG.  94. — The  perch,  Perca  flavescens.     A,  caught    by  a  pickerel;  B,  catching 
minute  animals  for  food;  C,  egg  string;  D,  capturing  a  dragon-fly  nymph. 

directed  teeth,  which,  though  not  used  for  chewing,  serve 
to  prevent  the  escape  cf  wriggling  animals. 

A  perch  is  a  very  good  swimmer  and  is  even  able  to  leap 
out  of  the  water  to  capture  small  animals.  Its  shape  is 
such  that  it  can  be  reasonably  successful  in  any  habitat  in 
fresh  water.  In  this  respect  it  differs  from  fishes  which  are 
restricted  to  certain  situations.  The  basses  and  sunfishes, 
for  example,  are  fitted  structurally  to  feed  among  aquatic 


PISCES  237 


vegetation;  suckers  have  their  mouths  turned,  downward 
for  feeding  from  the  bottom.  Perch  are  equally  at  home  in 
the  depths  of  lakes  or  in  the  dense  growth  of  plants  along- 
shore; also  thrive  in  ponds  and  streams.  They  can  feed 
from  the  surface,  strain  microorganisms  from  the  open 
water,  snatch  snails  and  the  larvae  of  insects  from  their 
retreats  among  plants  or  stones,  and  grub  about  in  the  soft 
bottom  mud  for  minute  worms  and  insect  larvae. 

In  its  sensory  equipment  for  finding  food  a  perch  presents 
a  rather  generalized  condition  when  compared  with  many 
other  fresh-water  fishes.  A  bullhead,  for  example,  pays 
little  attention  to  moving  objects  unless  they  touch  it,  but 
seeks  food  by  swimming  so  that  the  body  swings  from  side 
to  side.  The  "whiskers, "  or  barbels,  which  bear  taste 
organs,  thus  encounter  a  considerable  part  of  the  area  being 
explored.  If  a  barbel  touches  something  that  is  suitable 
for  food,  the  fish  at  once  turns  and  snaps  up  the  morsel.  A 
trout,  on  the  contrary,  depends  little  on  its  senses  of  taste 
and  smell  in  seeking  food,  but  is  quick  to  investigate  any 
small  moving  object.  It  chases  bubbles,  pursues  floating 
twigs,  and  examines  everything  that  moves  in  its  imme- 
diate vicinity,  thus  capturing  many  small  insects  and  other 
organisms.  The  bullhead  in  hunting  depends  primarily  on 
its  senses  of  taste  and  smell,  the  trout  uses  its  sense  of  sight 
more  than  any  other.  The  perch,  however,  has  enough 
versatility  to  feed  in  various  ways  and  therefore  has  greater 
opportunities  for  securing  food.  Its  acute  eyes  enable  it  to 
feed  on  the  small  moving  things  sought  by  the  trout;  its 
sense  of  taste,  though  not  distributed  so  widely  in  the  skin  as 
that  of  the  bullhead,  is  still  good  enough  to  be  of  value  in 
locating  and  judging  the  value  of  food.  The  presence  of 
food  substances  may  also  be  known  through  dilute  solutions 
given  off  in  the  water  which  are  perceived  by  the  sense  of 
smell.  If  anything  moves  that  promises  food  it  is  investi- 
gated and  perhaps  even  taken  into  the  mouth,  to  be  rejected 
again  if  not  suitable.  If  there  is  a  flavor  of  snail,  or  .worm, 
or  insect  in  the  water,  it  is  followed  to  its  source.  Small 


238  GENERAL  ZOOLOGY 

wonder,  then,  that  perch  usually  exceed  other  fishes  in  num- 
bers. They  are  far  too  versatile  to  be  kept  down  by 
scarcity,  or  other  variations  in  the  food  supply. 

When  food  enters  a  perch's  mouth,  it  is  taken  charge  of 
by  the  digestive  system,  and  passes  down  the  esophagus  to 
be  digested  and  absorbed  in  the  stomach  and  intestine. 
There  are  three  little  pockets,  or  caeca,  which  enter  the 
intestine  just  behind  the  stomach  and  help  to  increase  the 
surface.  The  liver,  pancreas,  and  intestinal  glands  pour 
their  secretions  into  the  intestine  to  be  mixed  with  the  food. 
Undigested  parts  collect  in  the  rectum  and  are  eliminated 
through  the  anus.  Absorbed  food  enters  the  circulatory 
system  and  in  the  blood  is  distributed  to  all  parts  of  the 
body.  The  blood  is  pumped  by  the  heart  to  the  gills,  where 
aeration  takes  place.  It  then  passes  through  the  four  aortic 
arches  (Fig.  105)  before  passing  to  the  head  and  to  the  great 
dorsal  aorta  which  distributes  it  to  all  parts  of  the  body. 
All  vessels  leading  away  from  the  heart  (arteries)  end  in 
minute  capillaries,  and  these  in  turn  open  into  veins  which 
carry  the  blood  back  to  the  heart,  to  be  pumped  round  the 
circuit  again.  In  short,  the  perch  has  a  " closed"  blood 
system  which  carries  food,  waste  products,  and  oxygen. 

The  important  organs  of  respiration  in  a  perch  are  the 
gills  and  the  air-bladder.  There  are  four  pairs  of  gills, 
each  bearing  a  double  row  of  soft  finger-like  branchial 
filaments,  through  the  walls  of  which  aeration  takes  place. 
As  water  passes  over  the  filaments,  the  blood  in  their 
capillaries  gives  up  its  waste  products  and  acquires  oxygen. 
The  air-bladder  serves  as  a  storage  reservoir  for  oxygen 
and  enables  a  perch  to  invade  stagnated  regions  without 
danger  of  suffocation.  It  contains  a  high  percentage  of 
oxygen  and  this  reserve  is  used  when  the  surrounding  water 
does  not  contain  a  sufficient  supply. 

Waste  products  accumulating  in  the  blood  are  carried  to 
the  kidneys  for  elimination  from  the  body.  A  kidney  con- 
sists of  a  vast  collection  of  uriniferous  tubules,  or  nephridia, 
which  are  individually  somewhat  like  those  described  in  the 


PISCES  239 

earthworm  (page  206),  but  are  not  metamerically  arranged. 
The  ureter  is  a  duct  which  carries  urine  from  the  kidney 
outside  the  body. 

Self-protection. — The  chief  enemies  of  perch  are  large 
predaceous  fishes  (the  pickerel,  bass,  dogfish)  and  man. 
Piscivorous  enemies  usually  lurk  in  the  shore  vegetation, 
and  capture  their  prey  by  fierce  and  sudden  rushes.  From 
these  perch  are  protected  to  some  extent  by  various  habits 
and  adaptations,  but  from  man,  with  his  nets,  hooks, 
spears,  and  traps,  there  is  no  escape. 

A  perch  is  well  protected  from  slight  injuries  by  its  cover- 
ing of  scales.  If  caught  by  a  pickerel,  or  some  other  enemy 
(Fig.  94,  A),  it  spreads  its  fins  so  as  to  inflict  painful  pricks 
when  the  bony  rays  are  touched.  There  are  also  sharp  saws 
along  the  edges  of  the  opercula  which  may  cause  severe 
cuts.  These  are  made  more  effective  by  spreading  the 
opercula  to  their  greatest  extent  when  danger  threatens. 
The  slimy  skin  and  the  backwardly  directed  scales  make  a 
perch  difficult  to  hold,  for  with  every  wriggle  the  body 
tends  to  slip  forward  a  little. 

Fishes  have  a  row  of  peculiar  sense  organs  extending 
along  either  side  of  the  body.  These  are  known  as  the  lat- 
eral lines,  and  are  believed  to  serve  for  the  perception  of  very 
slow  vibratory  movements  in  the  water,  such  as  would 
result  from  the  splashing  of  falling  bodies.  The  lateral 
line  organs  would  therefore  be  of  value  in  giving  notice  of 
disturbances  in  the  water. 

The  body  of  a  perch  is  "  countershaded  "  by  being  paler 
below,  so  that  when  light  strikes  it  from  above,  the  dark 
back  with  its  brighter  illumination  is  equal  to  the  lighter 
belly  in  the  shadow.  The  dark  vertical  bands  blend  readily 
with  the  lights  and  shadows  in  the  shore  vegetation,  or 
merge  with  the  ripples  on  the  surface  of  the  water  when 
seen  from  below.  A  perch,  like  other  fishes,  may  change 
its  color  somewhat  to  suit  the  background  on  which  it 
customarily  rests.  Such  changes  are  brought  about  by  the 
expansion  and  contraction  of  certain  pigmented  cells  in  the 


240  GENERAL  ZOOLOGY 

skin.  If  kept  on  a  black  background  for  a  month,  a  perch 
will  become  very  dark,  and  if  allowed  to  remain  on  a  white 
bottom,  will  assume  a  very  pale  color. 

Perch  have  considerable  power  of  regenerating  parts  of 
the  body  after  loss  through  accident  or  injury.  A  fin  may 
be  entirely  lost,  and  will  grow  again.  Many  fish  die,  how- 
ever, after  comparatively  slight  injuries  because  fish  moulds 
(Saprolegnia) ,  bacteria,  and  protozoans  gain  entrance  to  the 
body  through  the  wounds.  Perch  are  very  commonly 
infested  by  parasites.  Nearly  every  individual  examined 
will  be  found  to  have  the  white  "  bladders "  of  larval  tape- 
worms in  the  liver  or  among  the  visceral  organs.  These 
cysts  if  taken  into  a  bass  or  pickerel  will  grow  into  mature 
tapeworms.  The  perch's  muscles,  bile  ducts,  caeca,  and 
intestine  are  also  often  infested  with  trematodes. 

Perch  adjust  themselves  readily  to  conditions  in  the 
temperate  regions  of  the  earth.  In  winter  most  of  them 
retreat  into  the  depths  of  lakes  and  rivers,  where  the  tem- 
perature is  low  but  conditions  are  uniform  and  small 
aquatic  animals  furnish  a  plentiful  food  supply.  In  migrat- 
ing from  one  level  to  another  modifications  of  the  internal 
pressure  in  the  body  are  necessary,  and  such  adjustments 
are  brought  about  largely  by  slow  variations  of  the  gas  in 
the  air  bladder.  Perch  remain  active  throughout  the  year 
and  may  be  caught  on  a  hook  and  line  at  any  season. 

Race  Preservation. — Perch,  and  all  other  teleosts,  are  of 
separate  sexes.  Both  the  males  and  females  have  large 
gonads  (testes  or  ovaries)  and  are  nearly  mature  before 
cold  weather  begins,  During  the  winter  both  sexes  become 
fully  ripe  and  are  ready  to  begin  spawning  in  April  and 
May.  After  the  ice  goes  out  the  males  move  into  shallow 
water  and  are  soon  followed  by  the  females.  When  a  fe- 
male comes  inshore,  she  is  attended  by  one  or  more  males. 
As  she  deposits  her  "egg  string"  (Fig.  94,  C),  which  is  a 
crinkled  tube  of  jelly  containing  2000  or  3000  eggs,  the  male 
squirts  sperm  over  it  and  fertilization  is  thus  accomplished. 


PISCES 


241 


The  egg  strings  often  hang  on  sticks  or  stones  in  shallow- 
water. 

The  development  of  a  perch  egg  after  fertilization  follows 
the  usual  course  for  metazoans  (Fig.  64) ;  but  appears  to  be 
different  because  the  embryo  forms  on  one  end  of  the  egg. 
This  restriction  of  cell-division  and  embryo-formation  to 
the  germinal  disc  is  due  to  the  large  amount  of  yolk  which 
fills  the  rest  of  the  egg  and  which,  though  contained  in  the 
cytoplasm,  is  too  inert  to  permit  cell-division.  The  "fer- 
tilized" egg  (Fig.  95,  B)  is  a  zygote  formed  by  the  fusion  of 
an  ovum  (A)  and  a  sperm  cell.  The  nucleus  of  the  egg  cell 


F  G 

FIG.  95. — Embryonic  development  of  fish.  A,  unfertilized  egg;  gd,  germinal 
disc;  y,  yolk;  B,  zygote  formed  by  union  of  ovum  and  spermatozoon;  C,  D, 
cleavage;  E,  young  embryo  showing  neural  groove  at  left;  F,  showing  yolk  nearly 
nearly  overgrown  by  the  vascular  membrane  (blastoderm)  growing  out  from 
embryo;  G,  embryo  with  "yolk  sac;"  H,  young  fish,  just  hatched,  with  yolk  sac 
not  yet  absorbed. 

lies  at  one  end  in  an  area  of  protoplasm  called  the  germinal 
disc  (gd).  The  rest  of  the  egg  cell  is  filled  with  yolk 
material  (y).  The  sperm  cell  enters  the  germinal  disc,  and 
when  cleavage  follows  fertilization,  it  is  confined  to  that 
area.  As  the  embryo  grows  the  yolk  is  used  to  nourish 
it  (E,  F,  G).  When  the  little  perch  is  ready  to  hatch 
(G),  the  yolk  containing  part  of  the  egg  is  completely 
invested  by  a  vascular  membrane  and  forms  an  appendage, 
known  as  the  yolk  sac,  on  the  belly  of  the  young  fish.  After 
leaving  the  egg  envelopes,  the  young  perch  (H)  takes  no 
food  until  the  egg  sac  is  completely  absorbed. 


242  GENERAL  ZOOLOGY 

Perch  take  no  care  of  their  eggs,  and  may  even  devour 
them  or  their  young.  Other  species  of  fishes  often  follow 
perch  during  the  spawning  season  to  feed  upon  the  egg 
strings.  The  young  are  very  sluggish  until  the  yolk  sac  has 
been  absorbed,  then  quickly  learn  to  shift  for  themselves. 

GENERAL  REMARKS  ON  FISHES 

The  thirteen  thousand  species  of  fishes  now  in  existence 
show  a  wide  range  of  variation  in  structure  and  habits. 

The  morays  and  eels  have  long  snake-like  bodies;  the 
puffers  and  globefishes  are  more  or  less  spherical  in  form; 
the  goose-fishes  have  very  large  heads,  the  sunfishes  have 
large  bodies  and  very  small  heads;  most  fishes  are  well 
covered  with  scales,  but  the  catfish es  have  none.  The  fins 
vary  greatly  in  size,  structure  and  position.  There  is  end- 
less variation,  yet  all  fishes  are  recognizable  as  such,  for  they 
breathe  through  gills  and  are  without  fingers  or  toes  on 
their  paired  limbs. 

The  feeding  habits  of  fishes  are  often  very  interesting  and 
peculiar.  The  herring  eats  diatoms  and  other  microscopic 
organisms  which  it  strains  from  the  ocean.  One  scientist 
has  said  that  he  could  predict  the  herring  catch  by  the 
number  of  sunshiny  days — so  directly  is  the  growth  of 
diatoms  dependent  upon  light  from  the  sun,  and  so  im- 
t  portant  are  these  minute  plants  as  food  for  the  herring.  The 
black  swallower  takes  whole  fish  into  its  stomach  which 
greatly  exceed  its  own  size.  Various  fishes  feed  exclusively 
on  plants,  or  fish,  or  other  particular  foods.  Some  are  thus 
greatly  specialized,  others  will  eat  practically  anything 
they  encounter. 

The  only  method  of  locomotion  in  most  fishes  is  swimming 
by  using  the  fins,  but  there  are  many  exceptions.  The 
beach  skippers  along  the  shores  of  some  tropical  seas  seldom 
enter  the  water  but  hop  about  by  using  the  pectoral  fins  and 
tail.  The  climbing-perch^ is  not  only  able  to  come  on  land, 
but  is  said  to  climb  trees  and  catch  insects.  Flying  fishes 


PISCES 


243 


leave  the  water  and  sail  for  several  hundred  yards  before 
alighting.  The  great  majority  of  fishes  are  strictly  gill 
breathers,  but  there  are  also  some  exceptions  in  this  in- 
stance. The  dipncjans  are  able  to  breathe  either  through 
gills,  or  respire  air  directly  by  means  of  lungs.  Some  other 
fishes  gulp  in  air, and  breathe  through  the  lining  of  the 
throat  or  intestine. 

Breeding  habits  of  fishes  are  highly  variable  and  often 
show  extreme  adaptation.  A  cod  may  lay  over  nine  million 
eggs,  which  float  at  the  surface  of  the  ocean  and  receive  no 
care.  The  male  gaff-topsa'1-catfish  carries  the  eggs  laid  by 


A  B 

FIG.  96. — Fish  nests.      A,  stickleback;  B,  dogfish. 

the  female  in  his  mouth  until  after  they  hatch,  and  takes  no 
food  for  ninety  days.  The  male  pipefish  obtains  eggs  from 
a  female  and  carries  them  in  a  pouch  in  his  belly  until  after 
they  hatch.  In  some  species  of  sticklebacks  the  male  con- 
structs a  nest  (Fig.  96,  A).  In  this  he  induces  a  female  to 
lay  eggs,  and  watches  them  with  great  care.  Many  basses 
and  sunfishes  make  " nests"  by  clearing  a  shallow  basin  on 
the  bottom  in  which  eggs  are  deposited. 

Some  fishes  carry  on  extensive  seasonal  migrations.  The 
salmon,  shad,  sturgeon,  alewife,  and  striped  bass  spend  most 
of  their  lives  in  the  ocean,  but  enter  rivers  to  breed.  The 


244  GENERAL  ZOOLOGY 

king  salmon  on  the  Pacific  Coast  of  America  spawns  in 
November,  at  the  age  of  four  years  and  when  of  an  average 
weight  of  twenty-two  pounds.  It  spends  the  whole  of  the 
preceding  summer  in  ascending  the  rivers,  taking  no  food 
during  that  time,  but  living  on  its  stores  of  fat.  The  salmon 
that  run  up  the  Columbia  River  travel  a  thousand  miles, 
and  those  in  the  Yukon  journey  twice  as  far.  Eggs  are 
deposited  on  the  gravelly  bottoms  of  mountain  streams  >and 
the  fish ^which  have  made  the  long  journey  die  soon  after. 
The  young  salmon  spends  more  than  a  year  in  fresh  water 
before  passing  down  to  the  ocean  to  mature.  The  true 
eels  reverse  the  condition  found  in  the  salmon,  passing  their 
adult  life  in  fresh  or  brackish  water  and  returning  to  the 
ocean  to  breed. 

The  behavior  of  fishes  shows  great  diversity  in  instincts 
and  in  the  use  of  particular  sense  organs.  Many  species 
depend  largely  on  their  senses  of  taste  or  smell  for  procuring 
food,  others  use  their  eyes  exclusively^  and  are  quick  to 
take  any  small  moving  object  into  their  mouths.  Carp, 
mud  minnows,  trout  and  other  fishes  have  been  taught  to 
come  to  a  certain  spot  to  be  fed,  and  will  respond  to  signals, 
such  as  the  ringing  of  bells  or  the  display  of  objects  of  cer- 
tain colors.  Instincts  and  specialized  adaptations  are 
many,  modification  of  behavior  is  often  easy,  but  there  is 
little  or  no  power  of  reason. 

Early  zoologists  assumed  that,  because  fishes  live  in  water 
where  all  chemical  substances  that  could  be  smelled  or 
tasted  would  be  dissolved,  there  could  be  no  sharp  distinc- 
tion between  taste  and  smell,  and  some  went  so  far  as  to 
affirm  that  fishes  could  not  smell  at  all. '  Professor  Parker 
has  ^demonstrated,,  however,  that  fishes  smell  very  dilute 
solutionsan  the  nasal  pits,  and  taste  stronger  concentrations 
with  the  lips,  barbels,  or  other  parts  of  the  body.  They 
therefore  show  the  same  differentiation  in  the  chemical 
senses  as  that  in  terrestrial  vertebrates,  smell  being  used  for 
perceiving  greatly  diluted  substances  and  taste  for  stronger 
solutions.  - 


PISCES  245 

The  evolution  of  fishes  has  taken  place  during  the  ages 
still  represented  by  the  fossil  records  and  is  therefore  better 
known  than  that  of  any  phylum  of  invertebrates.  Though 
the  exact  ancestors  of  modern  fishes  are  unknown,  the 
general  course  of  events  is  pretty  well  recorded  in  the  rocks. 
Mariy  of  the  ancient  groups  of  fishes  are  now  completely 
extinct,  and  a  number  had  peculiar  characteristics  which 
died  out  with  them.  Some  had  the  body  covered  with 
heavy  armor,  and  were  provided  with  a  rather  flexible 
neck  joint,  so  that  the  head  could  nod.  Many  lacked 
teeth  and  had  no  paired  fins.  Among  fish-like  animals 
still  in  existence,  tlie  cyclostomes  and  sharks  were  the  first 
to  appear  in  the  past  (the  most  primitive  types  came  first), 
and  were  followed  somewhat  later  by  the  bony-fishes. 

Fisheries  are  of  great  economic  importance  in  all  parts 
of  the  world.  The  principal  species  serving  for  food  are: 
the  cod,  mackerel,  and  salmon.  In  the  year  1914  the  total 
catch  of  fish  brought  to  Boston  and  Gloucester  amounted 
to  162,589,220  pounds  and  had  a  iAie  of  $4,395,030. 
The  total  value  of  the  fishes  captured  in  the  United  States 
and  on  their  shores  is  enormous.  The  Bureau  of  Fisheries 
is  constantly  at  work  trying  to  increase  the  productiveness 
of  fishing  vand  improve  the  quality  of  its  output.  Fifty 
stations  are  maintained  for  propagation  and  study.  Dur- 
ing 1914,  4j288,757,800  eggs  and  young  fishes  were  planted 
in  suitable  waters. 


CHAPTER  XXIII 

SUBPHYLUM  VERTEBRATA,  CLASS 
AMPHIBIA 

Amphibians  have  soft  moist  skins,  generally  without  any 
sort  of  exoskeletal  structures.  This  distinguishes  them 
sharply  from  the  reptiles,  which  have  dry  scaly  skins.  On 
the  other  hand,  amphibians  cannot  be  confused  with  fishes 
because,  instead  of  fins,  they  usually  possess  paired  limbs 
with  toes  at  their  distal  extremities.  The  more  primitive 
amphibians  spend  their  lives  in  the  water  breathing  by 
means  of  gills,  but  specialized  forms,  like  the  frogs  and 
toads,  are  only  strictly  aquatic  during  larval  development 
and  spend  more  or  less  time  on  land  as  adults.  Thus, 
though  amphibians  are  typically  aquatic,  the  group  as  a 
whole  :.s  migrating  to  the  land,  and  a  few  species  have 
attained  a  truly  terrestrial  life.  This  is  perhaps  the  point 
of  greatest  interest  in  connection  with  amphibians — some 
live  much  like  fishes,  others  start  in  life  as  fish-like  tad- 
poles and  later  become  terrestrial. 

There  are  three  orders  in  the  class  Amphibia: 

1.  Apoda.     Degenerate,  limbless  amphibians  which  live  only  in  tropi- 
cal countries.    They  look  much  like  earthworms  and,  like  them,  bur- 
row about  underground.    The  eyes  are  very  degenerate  and  the  skin 
contains  skeletal  plates. 

2.  Caudata.     Salamanders;  aquatic  or  terrestrial  amphibians  which 
have  tails  when  adult. 

3.  Salientia.     Frogs  and  toads,  which  are  without  t-      in  the  adult 
condition. 

In  order  to  emphasize  the  differences  between  primitive 
and  specialized  amphibians,  representatives  of  two  orders 
will  be  considered — an  aquatic  salamander  (Necturus), 
and  the  common  garden  toad  (Bufo). 

246 


AMPHIBIA 


THE   MUD-PUPPY,   Necturus  maculosus  Rafinesque 


247 


North  America  is  fortunate  in  having  this  primitive 
salamander  within  its  boundaries,  mostly  through  the  Great 
Lake  Region.  Necturus  lives  among  the  rocks  along  the 
shores  of  lakes  and  streams,  and  is  commonly,  though 
incorrectly,  called  a  " lizard"  by  fishermen. 

Self-maintenance. — Necturus  (Fig.  97)  hunts  at  night 
for  crayfishes,  insects,  worms,  fishes  and  their  eggs  (C). 
Its  eyes  are  small,  poorly  developed,  and  of  little  or  no 


A  B  c  DC 

FIG.  97. — The    Mud   Puppy,    Necturus   maculosus.     A,    young;   B,   swimming; 
C,  catching  a  crayfish.     D,  eggs;  E,  larva  with  yolk  sac  nearly  absorbed. 

value  for  procuring  food,  but  the  sense  of  taste  is  acute  and 
generally  distributed  in  the  skin.  Necturus  is  able  to 
discover  small  animals  concealed  in  the  crevices  between 
rocks,  and  devours  them  whole.  The  jaws  are  armed  with 
double  rows  of  fine  pointed  teeth  for  holding  the  struggling 
victims  of  nocturnal  forays.  Necturus  usually  creeps  about 
slowly,  using  the  small  legs,  but  when  so  inclined  may 


248  GENERAL  ZOOLOGY 

swim  skillfully  by  making  undulatory  movements  with  its 
muscular  body  and  tail  (B). 

The  digestive  system  is  much  like  that  of  other  verte- 
brates, consisting  of  a  mouth,  pharynx,  oesophagus,  stom- 
ach, intestine,  liver  and  pancreas.  There  is  a  bladder 
opening  from  the  ventral  wall  of  the  intestine  which  re- 
ceives the  urine  brought  by  ducts  (ureters)  from  the  kid- 
neys. The  heart  has  three  chambers,  a  ventricle  and  two 
auricles.  One  auricle  is  filled  with  " impure"  blood  from 
the  larger  veins,  the  other  receives  aerated  blood  from  the 
gills.  The  structure  of  the  heart  is  such  that,  though  both 
kinds  of  blood  enter  the  ventricle  and  are  pumped  out 
together,  the  freshest  blood  goes  to  the  head  and  visceral 
organs,  while  most  of  that  charged  with  waste  products 
reaches  the  gills.  The  gills  are  tufts  of  filaments  through 
which  waste  gases  are  exchanged  for  oxygen.  Slender 
lungs  are  also  present  and  if  the  water  becomes  stagnant, 
Necturus  may  go  to  the  surface  and  gulp  in  air. 

Self-protection. — The  mud-puppy  is  protected  in  many 
ways  from  its  enemies.  It  is  acceptable  food  for  water 
snakes  and  other  predaceous  animals,  but  usually  escapes 
through  its  nocturnal  habits  and  protective  coloration. 
It  may,  like  other  cold-blooded  vertebrates,  change  its 
colors  somewhat  to  suit  the  background  on  which  it  rests. 
All  amphibians  have  poison  glands  in  the  skin,  and  the 
bitter  secretion  from  these  makes  the  mud-puppy  distaste- 
ful to  many  animals  which  might  otherwise  feed  upon  it. 
If  attacked  or  captured,  Necturus  will  bite  viciously,  but 
the  wounds  inflicted  are  not  poisonous,  as  many  fishermen 
believe. 

If  Necturus  loses  a  leg  or  a  piece  of  its  tail,  the  lost  parts 
are  regenerated  after  a  time.  The  process  of  regeneration 
and  growth  in  salamanders  has  been  the  subject  of  careful 
experimental  study,  and  some  fundamental  facts  have  been 
discovered.  For  example,  if  the  portion  of  an  embryonic 
salamander  eye  known  as  the  optic  cup  is  removed  and 
grafted  almost  anywhere  under  the  skin  of  the  head,  a 


AMPHIBIA  249 

complete  crystalline  lens  is  formed  in  the  new  situation. 
The  presence  of  optic  tissue  stimulates  the  ectoderm  to 
form  a  lens.  Metabolic  processes  have  much  more  latitude 
in  fishes  and  amphibians  than  in  warm-blooded  vertebrates 
—where  powers  of  regeneration  are  limited  and  growth 
processes  are  not  open  to  much  modification. 

The  mud-puppy  is  often  attacked  by  parasites  and  in- 
fectious diseases.  Fish-mould  (Saprolegnia) ,  a  fungus  which 
enters  the  body  through  slight  abrasions  and  destroys  the 
tissues,  is  the  most  important  of  these.  Another  common 
parasite  is  a  minute  trematode  worm  which  attaches  itself 
to  the  gills  and  sucks  blood. 

Necturus  remains  active  beneath  the  ice  throughout  the 
winter,  in  this  respect  resembling  fishes  and  some  other 
aquatic  animals  which  dwell  in  lakes  and  rivers.  The 
temperature  of  the  water  may  be  low,  but  never  falls 
below  freezing,  and  conditions  are,  therefore,  stable  enough 
to  permit  a  sluggish,  but  not  wholly  inactive,  existence. 

Race  Preservation. — Amphibians  are  always  dioecious, 
but  Necturus  males  are  much  like  the  females  in  appear- 
ance. Though  animals  are  usually  found  in  pairs  in 
autumn,  egg  laying  takes  place  in  May  and  June.  Each 
female  deposits  a  number  of  eggs  enclosed  in  little  cases 
and  attached  by  stalks  to  the  under  side  of  stones  (Fig. 
96,  D),  logs,  tin  cans,  and  other  objects  in  the  water,  where 
they  pass  through  the  usual  developmental  stages  (Fig. 
65).  The  whole  egg  separates  during  cleavage  (Fig.  115), 
but  at  one  pole  the  accumulation  of  yolk  retards  cell-forma- 
tion somewhat  and  the  cells  are,  therefore,  larger  there. 
Instead  of  forming  on  one  side  of  the  egg,  as  in  fishes,  the 
embryo  develops  entirely  around  the  yolk-containing  por- 
tion, but  there  is  a  large  yolk  sac  which  protrudes  from  the. 
ventral  side  of  the  developing  embryo  until  absorbed 
(Fig.  97,  E). 

The  young  (Fig.  97,  A)  are  striped  along  the  back,  and 
thus  differ  from  the  adults.  They  live  in  the  water  plants 
alongshore  where  they  are  effectively  concealed  by  their 


250  GENERAL  ZOOLOGY 

coloration.  Necturus  is  sometimes  called  a  "mature 
larva"  and  it  may  be  properly  compared  with  the  well- 
grown  tadpoles  of  many  other  amphibians.  The  gills  are 
functional,  but  lungs  have  developed  and  the  sexual  organs 
are  mature. 

GENERAL  REMARKS  ON  CAUDATA 

The  tailed  amphibians  show  an  interesting  transition 
from  aquatic  to  terrestrial  habits.  Necturus  is  entirely 
aquatic,  possessing  well-developed  gills  and  very  small 
lungs.  Siren,  which  lives  in  the  rivers  of  the  southeastern 
United  States,  starts  in  life  as  a  tadpole,  partly  loses  its 
gills,  and  then  has  them  enlarge  again  to  functional  size. 
Cryptobranchus  is  aquatic  throughout  life  and  breathes  by 
means  of  gills  which,  however,  are  covered.  Water  for 
respiration  enters  the  mouth  and  passes  out  through  a  pore 
on  either  side  of  the  neck.  Ambystoma  is  the  common 
" tiger"  salamander  throughout  the  eastern  and  central 
United  States.  It  is  of  particular  interest  because  certain 
species  in  the  genus  show  psedogenesis;  that  is,  they  be- 
come sexually  mature  while  in  the  larval  state,  and  an 
individual  with  gills  may  lay  fertile  eggs.  As  a  rule,  how- 
ever, the  adults  are  found  in  damp  situations  on  land, 
and  are  often  encountered  under  stones  and  logs,  or  in 
cellars.  Diemyctylus  is  the  beautiful  vermilion-spotted 
newt  of  New  England.  Its  life  cycle  requires  several  years 
for  completion.  During  the  first  three  years  it  is  a  green 
aquatic  larva;  then  migrates  to  the  land  and  becomes 
bright  orange  with  vermilion  spots.  After  a  period  of 
terrestrial  life,  the  newt  returns  to  the  water,  becomes 
green  in  color,  and  leads  an  entirely  aquatic  existence  while 
breeding.  Desmognathus  is  a  truly  terrestrial  salamander 
which  has  no  gills  or  lungs,  but  breathes  by  gulping  air 
into  the  throat,  which  is  very  vascular.  After  becoming 
mature  it  returns  to  the  water  to  breed.  Plethodon,  the 
red-backed  salamander,  does  not  enter  the  water  even  to 


AMPHIBIA  251 

breed.     It  lays  its  eggs  in  little  packets  under  logs  and 
stones,  and  guards  them  until  they  hatch. 

Considering  the  ontogeny  of  salamanders  and  the  habits 
of  various  existing  species,  the  law  of  biogenesis  (page  60) 
indicates  that  they  are  as  a  group  migrating  from  water 
to  land.  Nearly  all  begin  life  as  a  fish-like  tadpole  which 
breathes  by  means  of  gills,  but  in  the  adult  condition  there 
are  all  degrees  of  adjustment  to  terrestrial  life. 

THE  COMMON  TOAD,  Bufo  americanus  Le  Conte 

Salientians  in  general  are  highly  specialized.  They  start 
in  life  as  tadpoles,  but  are  tailless  when  mature  and  show 
many  evidences  of  specialization.  The  toad  is  a  typical 
representative.  It  is  specialized  in  its  marked  adjustment 
to  terrestrial  life,  but  shows  aquatic  affinities  in  its  habit 
of  annually  returning  to  the  water  to  breed. 

Self -maintenance. — A  toad  obtains  food  by  means  of 
its  flexible  tongue.  There  is  a  gland  in  the  roof  of  the 
mouth  which  forms  a  very  sticky  secretion.  When  the 
tongue  is  suddenly  protruded,  it  adheres  to  the  food  and 
draws  the  morsel  back  into  the  mouth  (Fig.  98,  B).  The 
tongue  is  extended  by  suddenly  filling  a  lymph  space  at 
its  base.  Toads  depend  chiefly  on  their  sense  of  sight  for 
procuring  food  and  will  snap  at  almost  any  moving  object. 
This  habit  is  general  among  salientians,  and  is  taken  ad- 
vantage of  by  fishermen  who  use  a  bright-colored  rag  as 
bait  for  catching  frogs.  A  toad  has  no  teeth  and  insects 
captured  are  swallowed  at  once  without  any  preliminary 
chewing.  If  some  distasteful  object  is  snapped  up,  it  is 
quickly  spit  out,  even  after  it  has  passed  down  into  the 
stomach. 

The  food  habits  of  toads  make  them  of  considerable 
economic  importance.  A  recent  estimate  by  a  scientific 
expert  in  the  Department  of  Agriculture  places  the  annual 
value  of  a  single  individual  in  a  garden  at  $19.44.  In  cities 
also  a  toad  may  do  great  good  by  feeding  at  night  on  the 
insects  about  street  lights. 


252 


GENERAL  ZOOLOGY 


A  toad  has  two  methods  of  locomotion — hopping  and 
walking.  The  whole  body  is  adapted  structurally  for 
progression  by  leaping,  and  in  moving  about  that  method  is 
usually  employed,  but  at  close  quarters  a  toad  is  able  to 
stalk  its  food  as  cautiously  as  a  lynx.  An  active  insect, 
however,  is  often  secured  by  leaping  upon  it  from  a  distance 
of  two  or  three  feet. 


A  Be 

FIG.  98. — Salientia.  A,  Wood  frog  and  green  frog;  B,  the  garden  toad, 
Bufo  americanus — male  singing,  catching  fly,  egg  string,  tadpoles;  C,  tree  frog 
and  cricket  frog. 

The  metabolic  processes  of  a  toad  are  much  like  those  of 
other  cold-blooded  vertebrates.  The  digestive  and  excre- 
tory organs  resemble  those  of  Necturus.  The  circulatory 
and  respiratory  systems  are,  however,  adjusted  to  life  on 
land.  The  blood-vessels  which  supply  the  gills  during 
larval  life  are  diverted  to  the  lungs  and  skin  when  terres- 


AMPHIBIA  253 

trial  habits  are  assumed  (Fig.  105,  B).  Most  salientians 
breathe  as  much  through  their  soft  moist  skins  as  'through 
the  lungs.  On  this  account  they  are  able  to  endure  pro- 
longed submergence;  the  blood  being  aerated  through  the 
skin  from  the  surrounding  water. 

Self-protection. — Though  toads  have  bitter,  poisonous 
secretions  in  the  skin,  they  are  eagerly  eaten  by  snakes, 
owls,  and  other  animals.  They  escape  from  many  enemies, 
however,  by  doing  most  of  their  hunting  in  the  early  morn- 
ing or  at  twilight,  and  hiding  in  their  holes  during  the  day. 
Hopping  is  a  somewhat  erratic  method  of  locomotion  and 
toads  are  hence  often  able  to  elude  a  pursuer.  Their  colors 
also  render  them  relatively  inconspicuous,  and  may  change 
somewhat  to  match  different  backgrounds. 

Salientians  have  less  power  of  regeneration  when  adult 
than  salamanders.  If  a  foot  is  lost,  it  does  not  grow  again, 
but  the  leg  remains  a  deformed  stump.  The  tadpoles  of 
frogs  and  toads  are  rather  remarkable  for  their  ability  to 
replace  lost  parts,  and  continue  to  grow  after  serious  in- 
juries. Eight-  and  ten-legged  frogs,  two-tailed  tadpoles, 
and  other  monstrosities  have  been  produced  by  splitting 
embryonic  structures.  It  is  also  possible  to  form  composite 
tadpoles  by  grafting  parts  of  different  individuals  together. 

On  account  of  their  terrestrial  life  toads  cannot  remain 
active  during  the  winter  like  the  strictly  aquatic  am- 
phibians. At  the  approach  of  cold  weather,  they  burrow 
into  the  ground  below  frost  and  remain  in  a  torpid  condi- 
tion. During  hibernation  metabolism  is  very  slow  and 
many  activities  are  dormant;  a  toad  may  be  cut  in  pieces 
without  giving  any  sign  of  life. 

Race  Preservation. — Toads  show  one  great  advance  in 
complexity  of  mating  instincts  when  compared  with  a 
salamander — they  make  use  of  sounds.  In  order  to  insure 
the  continued  existence  of  any  race  of  animals,  the  eggs 
must  be  fertilized  and  any  adaptation  which  tends  to  make 
this  more  certain  is  therefore  desirable.  In  spring  when 
the  male  toads  enter  the  ponds  and  utter  their  shrill  notes 


254  GENERAL  ZOOLOGY 

(Fig.  97)  they  are  giving  notice  to  every  mature  female 
within  hearing  of  their  presence  and  ability  to  fertilize 
eggs. 

During  mating  the  male  clasps  the  female  with  his  front 
legs  and  squeezes  the  eggs  from  her  body.  As  eggs  emerge, 
sperm  is  discharged  over  them  and  fertilization  takes  place 
in  the  water.  The  clasping  reflex  of  the  male  is  controlled 
in  the  spinal  cord  and  is  very  strong  during  the  breeding 
season.  A  male  while  clasping  may  have  his  head  removed 
and  the  whole  of  the  body  behind  the  front  legs  cut  away 
without  letting  go  his  grip;  the  front  legs  continue  to  hold 
on  until  they  die. 

The  eggs  of  toads  are  laid  in  long  strings  of  jelly  which 
protect  them.  They  are  usually  deposited  at  night  and  a 
single  female  produces  from  four  to  ten  thousand  each 
spring.  Tadpoles  hatch  from  the  jelly  about  four  days 
after  the  laying  of  the  eggs.  At  first  they  have  short  tails 
and  a  protruding  yolk  sac  on  the  belly.  They  have  no 
mouth,  but  cling  to  aquatic  vegetation  by  means  of  a 
sucker  beneath  the  head  until  the  yolk  is  absorbed.  On 
the  tenth  day  the  mouth  is  well  formed  and  the  tadpoles 
begin  to  feed  on  water  plants.  The  gills,  which  are  feathery 
tufts  on  the  sides  of  the  head  at  the  time  of  hatching,  are 
gradually  covered  by  the  backward  growth  of  a  membrane 
(the  operculum)  from  the  head,  and  a  well-grown  tadpole 
has  only  one  exit  for  the  respiratory  water  through  a  pore 
on  the  left  side. 

The  legs  grow  out  as  little  buds  which  later  develop  toes 
at  their  distal  ends.  The  front  pair  are  formed  inside  the 
opercular  cavity  and  do  not  appear  externally  until  the 
tadpole  is  nearly  ready  to  leave  the  water.  As  a  tadpole 
grows  older  it  shows  a  greater  fondness  for  animal  food, 
and  the  alimentary  canal  accordingly  grows  shorter.  It  is 
a  general  rule  among  animals  that  vegetable  food  is  associ- 
ated with  great  length  of  digestive  tract,  and  carnivorous 
diet  with  a  short  enteron.  The  teeth  in  the  tadpole  also 
change  and  the  earlier  chitinous  exoskeletal  plates  associ- 


AMPHIBIA  255 

ated  with  plant  food  are  lost  (in  frogs  they  are  replaced  by 
bony  teeth). 

Tadpoles  hatched  from  eggs  laid  about  the  first  of  June 
will  transform  into  young  toads  about  the  middle  of  July. 
As  a  tadpole  nears  the  end  of  its  sojourn  in  the  water,  the 
gills  grow  smaller  and  lungs  are  developed.  Frequent  trips 
are  made  to  the  surface  of  the  water  and,  after  the  front 
legs  become  functional,  much  time  is  spent  on  land,  just 
out  of  the  water.  The  tail  becomes  soft  and  flabby,  grows 
gradually  shorter,  and  finally,  some  fine  day,  shrivels  up 
completely.  Then  the  tadpole  is  no  more,  but  a  little  toad 
hops  away  to  hide  in  the  grass  and  hunt  for  small  insects. 
Often  minute  toads  migrate  from  ponds  in  great  armies 
after  a  rain,  and  ignorant  people  therefore  believe  that  they 
rained  down.  The  delicate  creatures  are  not  yet  fully 
adjusted  to  life  on  land  and  are  unusually  active  when  the 
air  is  moist. 

If  we  apply  the  Law  of  Biogenesis  to  the  development  of 
a  toad,  it  leads  to  the  same  conclusion  as  in  the  case  of  a 
salamander.  The  ancestors  of  amphibians  were  aquatic 
fish-like  vertebrates  which  later  developed  lungs  and  left 
the  water.  Most  salientians  are  more  markedly  terrestrial 
than  caudate  amphibians,  and  show  more  striking  adapta- 
tions for  life  on  land.  Some  species  even  live  in  deserts 
where  they  pass  the  dry  season  in  a  condition  of  aestivation, 
but  most  amphibians  are  limited  to  rather  moist  situations 
on  account  of  their  soft  skins.  They  are  for  the  most  part 
confined  to  shady  nooks  in  the  shelter  of  vegetation  and 
seldom  hunt  in  the  glare  of  a  noonday  sun.  They  are  not 
as  well  adapted  to  the  land  as  reptiles,  birds,  and  mammals. 

GENERAL   REMARKS   ON  SALIENTIANS 

The  commonest  frogs  throughout  the  United  States  are 
the  leopard  frogs,  Rana  pipiens,  and  related  species.  The 
green  frog,  which  is  often  brown  or  mottled,  but  may 
always  be  recognized  by  its  very  large  tympanic  mem- 


256  GENERAL  ZOOLOGY 

branes,  is  also  quite  common  (Fig.  98).  The  largest  and 
most  aquatic  species  are  the  bull-frogs;  some  individuals 
attain  a  body  length  of  seven  or  eight  inches.  The  wood 
frogs  are  adapted  to  terrestrial  life,  and  may  spend  a  large 
part  of  the  year  at  considerable  distances  from  water. 

The  tree-frogs  (C)  usually  live  among  vegetation.  They 
have  sucking  discs  at  the  tips  of  the  toes,  which  enable 
them  to  climb  with  ease.  They  also  possess  remarkable 
ability  to  change  their  colors  to  match  the  surroundings. 
The  little  brown  cricket  frog  of  the  United  States  is  a 
degenerate  among  tree-frogs  which  spends  most  of  its 
life  in  or  near  the  water  and  has  only  rudimentary  discs 
at  the  ends  of  its  toes.  A  very  specialized  tree-frog  in 
Java  is  able  to  sail  from  tree  to  tree  by  using  the  spread 
webs  between  the  toes.  It  does  not  go  to  ponds  to  breed, 
but  makes  a  suspended  nest  inside  a  rolled  leaf. 


CHAPTER  XXIV 
SUBPHYLUM  VERTEBRATA,  CLASS  REPTILIA 

Reptiles  are  cold-blooded,  scaly  vertebrates,  which 
breathe  by  means  of  lungs.  The  body  is  never  soft  and 
slimy  as  in  amphibians,  but  covered  with  a  dry  skin  which 
conserves  the  moisture  within  and  permits  existence  on 
land  without  danger  of  dessication.  The  representatives 
of  this  class  are  typically  terrestrial ;  even  the  marine  turtles 
and  sea  snakes,  which  spend  their  lives  in  the  open  ocean, 
always  return  to  the  land  to  breed.  With  the  exception 
of  the  snakes,  limbless  lizards,  and  certain  other  forms  in 
which  the  legs  are  somewhat  degenerate,  reptiles  have 
claws  at  the  tips  of  the  toes.  There  are  four  orders  of 
living  reptiles  and  six  times  as  many  which  existed  in  the 
past  but  are  now  extinct.  The  orders  of  existing  reptiles 
are  as  follows: 

Order  1.  Squamata.  Body  elongated  and  covered  with  small  scales, 
anus  slit-like  and  extending  across  the  body. 

Suborder  1.  Sauria.  Limbs,  eyelids,  and  external  ear  opening 
usually  present.  Lizards. 

Suborder  2.  Serpentes.  Limbs,  eyelids,  and  external  ear  opening 
absent.  Snakes. 

Order  2.  Rhynchocephalia.  Represented  by  a  single  species, 
Sphenodon  punctatum,  a,  reptile  resembling  the  lizards,  but  possessing 
a  large  eye-like  organ  in  the  center  of  the  forehead.  This  animal  is 
found  only  in  New  Zealand. 

Order  3.  Testudinata.  Body  short,  broad,  and  enclosed  between 
two  (dorsal  and  ventral)  shields.  Turtles,  tortoises. 

Order  4.  Crocodilia.  Body  elongate  and  adapted  to  aquatic  life; 
limbs  present;  anus  a  longitudinal  slit.  Alligators,  caimans,  crocodiles. 

Before  discussing  the  different  orders,  the  activities  of  a 
little  lizard  which  inhabits  the  Eastern  United  States  will 
be  considered  in  detail. 

257 


c= 


258  GENERAL  ZOOLOGY 


THE  BLUE-TAILED  SKINK,  Eumeces  quinquilineatus 
Linnaeus 


Self -maintenance. — This  little  lizard  hunts  among  vege- 
tation for  insects,  which  constitute  its  principal  food.  It 
sometimes  eats  bird's  eggs,  young  field  mice,  or  other 
animals  of  suitable  size.  It  is  well  fitted  to  seek  and  cap- 
ture food.  Its  sight  is  very  acute.  The  sense  of  taste  is 
also  well  developed,  for  ill-flavored  substances  are  often 
spit  out  after  being  taken  into  the  mouth.  The  claws  on 
the  ends  of  the  toes  enable  the  skink  to  run  up  and  down 
the  trunks  of  trees  with  great  swiftness.  Though  the 
mouth  is  provided  with  teeth  for  holding  prey,  the  food 
is  not  chewed,  except  enough  to  kill  it,  before  being 
swallowed. 

The  digestive  organs  of  the  skink  resemble  those  of  other 
vertebrates,  but  the  respiratory,  circulatory  and  excretory 
systems  are  better  adjusted  to  terrestrial  life  than  those 
of  the  land  amphibians.  There  is  no  respiration  except 
through  the  lungs;  the  structure  of  these  organs  and  their 
associated  parts  is  more  specialized  than  are  those  of  an 
amphibian.  There  are  little  chambers  on  the  inner  wall 
of  the  lungs  which  increase  the  surface  exposed  and  no 
large  branches  of  the  blood-vessels  supply  the  skin.  Air  is 
drawn  into  and  expelled  from  the  lungs  largely  by  move- 
ments of  the  ribs,  whereas  frogs  and  toads  have  no  ribs, 
but  force  air  in  by  gulping  movements  of  the  throat.  The 
chief  excretion  product  in  a  reptile  is  uric  acid,  which  is 
thrown  off  as  a  solid — in  fact,  is  insoluble  in  water — and 
is  therefore  more  desirable  for  a  terrestrial  animal  (which 
needs  to  conserve  its  water)  than  urea,  which  requires 
liquid  for  its  elimination. 

As  the  skink  grows  the  "skin"  is  shed  from  time  to 
time.  The  whole  epidermis  does  not  come  away  in  one 
piece,  as  in  many  snakes,  but  sloughs  off  in  patches. 
Preparation  is  made  in  the  skin  before  shedding  occurs 
.and  a  softening  takes  place  in  a  particular  layer  of  the 


REPTILIA 


259 


epidermis,  so  that  the  outer  part  may  be  pushed  off  with- 
out injuring  what  remains. 

Self -protection. — The  skink  is  very  shy  and  very  swift. 
When  basking  in  the  sun  or  chasing  insects,  it  is  always 
ready,  if  danger  threatens,  to  scuttle  away  to  some  safe 
retreat  which  it  knows  about.  It  hunts  entirely  by  day 
and  when  inactive  usually  hides  in  some  crevice  with  only 
its  head  protruding.  The  watchful  eyes  give  warning  of 
any  disturbance,  and  the  skink  is  hard  to  catch.  If 
grasped  by  an  enemy,  however,  it  is  still  not  beyond  hope 


FIG.  99. — The  Blue-tailed  Skink,  Eumeces  quinquilineatuA.     A,  adult  male  catch- 
ing insect;  B,  young;  C,  female  with  her  eggs  in  a  cavity  in  a  rotten  log. 

of  escape.  The  tail  may  be  cast  off  and  left  with  the 
enemy  while  its  owner  runs  away.  Most  lizards  shed  the 
tail  very  readily  and  no  permanent  harm  results,  for  it 
grows  again  after  a  time. 

A  skink's  colors  change  with  age.  In  youth  the  body  is 
jet  black  with  five  longitudinal  yellow  stripes  on  the  back 
and  a  bright  blue  tail.  After  three  or  four  years  the  colors 
become  dull,  and  a  mature  individual  may  lose  its  stripes 
completely.  An  adult  male  has  a  bright  copper-red  head 
and  a  brown  body;  a  female  is  usually  brown  with  light 


260  GENERAL  ZOOLOGY 

longitudinal  stripes.  In  general  the  colors  are  suitable  for 
the  activities  of  different  kinds  of  individuals,  and  skinks 
are  therefore  protected  from  enemies  at  all  ages. 

There  is  one  defect  in  the  skink's  means  of  defense.  A 
reptile  of  any  kind  cannot  remain  active  at  low  tempera- 
tures. During  cool  weather,  therefore,  a  dormant  skink, 
if  its  hiding  place  is  discovered,  makes  a  fine  meal  for  any 
prowling  warm-blooded  animal. 

Race  Preservation. — Skinks,  like  other  reptiles,  are  of 
separate  sexes.  In  the  species  under  discussion  the 
mature  males  are  easily  recognized  by  their  bright  red 
heads.  In  the  northern  states  they  mate  with  the  females 
in  June,  fertilization  taking  place  within  the  female's  body 
before  the  shell  is  formed  about  the  eggs.  This  is  an 
improvement  over  the  method  of  fertilization  prevalent 
in  amphibians,  where  the  sperm  and  egg  cells  are  usually 
discharged  into  the  water.  When  the  eggs  are  laid,  early 
in  July,  segmentation  has  already  begun.  They  are  de- 
posited in  little  cavities  in  the  rotten  wood  of  old  stumps, 
and  the  mother  remains  with  them  until  they  hatch  (Fig. 
99,  C).  In  August  the  young  skinks  leave  the  nest~and 
start  out  for  themselves. 

Though  the  development  of  reptiles  is  similar  to  that 
of  amphibians,  it  shows  greater  specialization  for  terres- 
trial life.  A  skink,  or,  any  other  reptile,  always  lays  its 
eggs  on  land,  whereas  amphibians  usually  deposit  theirs 
in  water.  The  gills  in  an  embryo  skink  are  absorbed  long 
before  the  egg  hatches  and  are  never  functional  as  breath- 
ing organs.  A  lizard  also  shows  greater  ability  to  adjust 
itself  to  its  surroundings;  in  other  words — is  less  stupid 
than  a  frog  or  toad,  and  even  has  some  glimmerings  of 
intelligence.  If  a  skink  encounters  a  distasteful  insect, 
like  a  hornet  or  an  ill-flavored  caterpillar,  it  tries  to  eat 
it  the  first  time,  but  after  that  avoids  all  insects  of  similar 
appearance.  It  can  learn  in  its  dull  way  that  a  certain 
color  or  form  is  associated  with  disagreeable  experiences. 


REPTILIA  261 

GENERAL  REMARKS  ON  THE  ORDER  SQUAMATA 

Lizards  are  clearly  more  generalized  animals  than  snakes. 
Most  of  them  possess  four  well-developed  legs  with  toes, 
though  there  are  a  few  which  agree  with  snakes  in  lacking 
any  external  indication  of  appendages  in  the  adult  condi- 
tion. Snakes  are  specialized  in  the  absence  of  appendages, 
the  structure  of  their  jaws,  and  in  other  particulars.  Yet, 
despite  the  divergences  between  snakes  (Serpentes)  and 
lizards  (Sauria) ,  their  fundamental  plans  of  structure  are 
so  similar  that  they  are  placed  by  systematic  zoologists  in 
one  order.  , 

It  cannot  be  said,  however,  that  many  lizards  are  not 
specialized;  some,  indeed,  are  highly  adapted  to  peculiar 
modes  of  life.  The  limbless  species  all  burrow,  and  are 
seldom  seen  above  the  surface  of  the  ground.  The  flying- 
dragon,  Draco  volans,  has  thin  lateral  expansions  on  the 
body  which  are  supported  by  the  ribs  and  furnish  enough 
Surface  so  that  the  animal  can  sail  from  tree  to  tree.  The 
chameleons  have  prehensile  feet  and  tail,  hence  are  able  to 
climb  with  great  sureness,  and  their  remarkably  extensible 
tongue  enables  them  to  snap  up  insects  among  the  trees. 
Their  unusual  ability  to  change  the  colors  of  their  skins 
is  also  noteworthy.  The ," horned  toads"  are  adapted  to 
life  in  the  desert.  They  are  spiny,  burrow  to  escape  ex- 
treme heat  or  cold,  and  have  other  adaptations  which 
enable  them  to  live  in  arid  localities. 

The  largest  lizards  in  America  are  the  iguanas,  which 
range  from  the  southern  United  States  southward.  They 
attain  a  length  of  six  feet,  and  are  often  eaten  by  man. 
They  eat  vegetation,  insects,  bird's  eggs,  and  animals  up 
to  the  size  of  a  half-grown  rabbit.  Through  the  southern 
states  the  little  aaoles  are  very  common  among  plants. 
They  are  easily  tamed  and  are  often  sold  by  curio  dealers. 
The  only  poisonous  lizards  known  live  in  the  southern 
United  States  and  Mexico.  These  Gila  monsters  (Helo- 
derma)  are  sluggish  but  powerful  creatures  with  bands  of 


262  GENERAL  ZOOLOGY 

bright  red  and  black  around  the  body.  They  have  a 
poisonous  spittle^nd  bite  viciously  when  molested.  When 
their  bodies  are  well  nourished  the  tails  become  swollen 
with  stored  fat,  and  they  can  then  live  for  some  time  with- 
out food. 

Snakes  are  not  only  without  limbs  but  show  various 
other  adaptations  correlated  with  their  slender  forms. 
One  lung  is  usually  rudimentary,  the  visceral  organs  are 
peculiarly  arranged,  and  there  is  no  urinary  bladder.  The 
two  halves  of  the  lower  jaw  are  connected  by  an  elastic 
ligament,  and  are  loosely  fastened  to  the  skull.  This 
arrangement  makes  it  possible  for  a  snake  to  swallow 
animals  which  greatly  exceed  its  own  body  in  diameter. 
Snakes  move  by  waving  the  ventral  scales  backward  and 
wiggling  the  body.  Muscles  lead  from  the  scales  to  the 
ribs,  which  help  materially  in  locomotion.  There  is  great 
variation  in  the  size  of  snakes — a  python  may  have  a 
length  exceeding  thirty  feet  and  weigh  over  three  hundred 
pounds;  some  of  the  burrowing  snakes  are  less  than  six 
inches  in  length  and  no  thicker  than  a  goose  quill. 

Snakes  may  be  viviparous  or  oviparous;  that  is,  the 
young  of  some  species  are  born  and  those  of  others  hatch 
from  eggs.  The  garter  snakes  bear  their  young  in  such 
condition  that  they  are  soon  able  to  shift  for  themselves. 
The  milk  snake  "lays  eggs  in  holes  in  the  ground. 

Though  there  are  many  venomous  snakes,  the  harmless 
species  greatly  outnumber  them.  Of  the  one  hundred 
and  eleven  species  in  the  United  States  less  than  twenty 
are  poisonous.  The  venomous  snakes  have  a  poison  gland 
on  either  side  of  the  head  which  is  connected  with  a  grooved 
or  hollow  fang  in  the  mouth;  the  non-poisonous  species 
have  the  jaws  armed  with  sharp  backwardly  directed  teeth 
which  hold  the  food  and  help  in  swallowing.  There  are 
two  types  of  poison  fangs— grooved  and  tubular.  The 
former  is  more  primitive,  and  the  hollow  type  has  ap- 
parently been  derived  from  it  by  a  closing  over  of  the 
groove.  The  grooved  fang  is  usually  fastened  immovably 


REPTILIA  263 

to  the  jaw;  but  the  hollow  fang  is  folded  against  the  upper 
jaw  when  not  in  use  and  becomes  erect  when  the  mouth  is 
opened  to  strike.  Snakes  continually  protrude  the  forked 
tongue  when  they  are  moving  about  and  this  habit  has 
often  led  to  the  belief  that  the  tongue  is  the  fang  or  "sting," 
an  idea  which  is  of  course  erroneous. 

In  the  United  States  there  are  only  five  kinds  of  poisonous 
snakes:  coral  snakes,  water  moccasin,  copperhead,  rattle 
snakes,  and  opisthoglyphs.  Two  species  of  coral  snakes 
(Flaps)  are  found  in  the  south.  They  are  banded  with 
red,  black,  and  yellow,  and  have  grooved  fangs.  Four 
species  of  opisthoglyphs  are  found  along  the  southern 
border  of  the  United  States.  They  are  small  snakes  with 
grooved  fangs  which  are  set  far  back  in  the  mouth.  They 
cannot  readily  inject  poison  by  striking  quickly  at  an 
animal,  but  must  take  the  prey  into  the  mouth  to  kill  it. 
The  other  three  types  range  farther  north,  have  hollow 
fangs,  and  belong  to  the  group  of  venomous  snakes  known 
as  pit  vipers.  In  these  there  is  a  well-defined  depression 
on  either  side  of  the  head  between  the  nostril  and  the  eye. 
The  water  moccasin  or  cotton-mouth  (Ancistrodon  pisci- 
vorus)  is  a  semi-aquatic  serpent  which  frequents  the 
lagoons  and  sluggish  waterways  in  the  southeastern  states. 
It  averages  about  four  feet  in  length  but  may  reach  six. 
The  body  is  very  stout  and  heavy,  with  an  abruptly  taper- 
ing tail  and  a  chunky,  ugly  head.  The  copperhead  (Ancis- 
trodon contorlrix)  is  found  east  of  the  Mississippi  River  from 
Massachusetts  to  Florida.  It  frequents  forests  and  planta- 
tions, hiding  among  fallen  leaves  which  it  closely  resembles 
in  color.  There  are  nineteen  species  of  rattle  snakes  and 
the  majority  of  them  are  found  in  the  United  States  and 
Mexico.  The  diamond-back  (Crotalus  adamateus)  of  the 
/southeastern  states  is  the  largest  and  most  deadly  of  our 
native  serpents.  The  rattle  is  an  unique  organ  among 
snakes. '  The  little  bells  which  compose  it  are  formed  each 
time  the  skin  is  shed,  and  are  not  closely  indicative  of  the 
snake's  age. 


264 


GENERAL  ZOOLOGY 


ORDER  3.    TESTUDINATA  (CHELONIA) 

The  turtles  and  tortoises  (Fig.  100)  are  clearly  distin- 
guishable from  all  other  reptiles  by  the  beak-like,  toothless 
jaws  and  by  the  shell  covering  the  body.  The  turtles  are 
aquatic,  some  species  never  going  on  land  except  to  breed, 
and  the  tortoises  are  terrestrial.  All  chelonians  lay  eggs, 
which  are  buried  in  shallow  excavations  on  sandy  beaches 


Fio.  100.— Testudinata.     A,  the  common  painted  turtle;  B,  Soft-shell  turtle; 
C,  Snapper;  D,  turtle  eggs;  E,  terrapin. 

or  on  hillsides  (D).  Their  food  varies  greatly:  the  giant 
tortoises  of  the  Galapagos  Islands  subsist  on  cactus;  cer- 
tain marine  turtles  eat  molluscs,  fishes,  and  seaweeds;  the 
fresh- water  species  feed  upon  vegetation,  molluscs,  cray- 
fishes, insects,  fish,  frogs,  snakes,  birds,  mice,  rabbits,  etc. 
In  the  interior  of  the  United  States  there  are  a  number 
of  species  of  chelonians.  The  snappers  (C)  are  rough- 
backed  ferocious  animals.  The  common  species  in  the 
north  (Chelydra  serpentina)  does  not  weigh  over  forty 
pounds,  but  the  southern  alligator  snapper  (Macrochelys 


REPTILIA  265 

lacertina)  reaches  a  hundred  and  forty.  The  painted,  or 
"mud,"  turtles  (A)  are  the  commonest  chelonians  through- 
out the  country  and  may  often  be  seen  sunning  themselves 
on  logs  along  swampy  shores.  The  soft-shelled  turtles 
(PlatypeltiSj  B)  are  remarkable  for  their  leathery  covering 
and  the  extreme  length  of  their  necks. 

Through  the  middle  states  Blanding's  turtle  is  common 
along  ponds  and  streams.  It  is  a  "semi-box"  turtle;  that 
is,  there  is  a  hinge  across  the  ventral  part  of  the  shell,  so 
that  it  may  be  closed  over  the  retracted  head  and  front 
legs.  It  spends  more  time  out  of  water  than  most  turtles, 
and  is  often  found  wandering  in  fields  or  woods.  In  the 
southeastern  states  lives  the  terrapin,  Terrapene  Carolina 
(E),  which  is  a  true  box  tortoise.  It  has  the  ventral  plate 
so  hinged  that  it  completely  closes  the  shell.  The  terrapin 
lives  in  woods  and  does  not  enter  the  water;  feeding  on 
berries,  earthworms,  and  insects.  There  are  a  number  of 
species  of  giant  tortoises  inhabiting  islands  of  the  Pacific 
and  Indian  Oceans.  They  never  enter  the  water,  and  feed 
on  cacti,  grass,  leaves,  and  fruits. 

Along  the  shores  of  warm  seas  the  large  marine  turtles 
are  regularly  caught  for  the  market,  being  harpooned  in  the 
open  ocean  or  captured  while  ashore  for  breeding.  The 
loggerhead  turtle  may  reach  a  length  of  four  feet  and  weigh 
five  hundred  pounds.  The  green  turtle  is  somewhat 
smaller  (150  pounds),  but  is  more  highly  esteemed  for  the 
table.  Almost  any  turtle  is  suitable  as  food  for  man,  but 
the  marine  turtles  and  tortoises  are  most  generally  used. 
The  Bureau  of  Fisheries  has  demonstrated  by  recent  experi- 
ments that  the  terrapin  may 'be  reared  successfully  for  the 
market  on  small  farms.  Most  of  the  fresh-water  turtles 
do  considerable  damage  by  destroying  fishes  and  small 
aquatic  animals  which  might  serve  as  fish  food. 

ORDER  4.     CROCODILIA 

The  crocodiles  and  their  relatives  resemble  the  lizards  in 
form,  but  attain  a  much  greater  size  and  are  adapted  to 


266  GENERAL  ZOOLOGY 

aquatic  conditions.  The  nostrils  and  ears  can  be  closed, 
and  there  is  a  valve  at  the  back  of  the  mouth  so  that  food 
may  be  captured  under  water  without  danger  of  filling  the 
lungs.  The  teeth  are  set  in  bony  sockets,  and  there  are 
bony  plates  beneath  the  scales  in  the  skin.  The  heart  has 
four  chambers  (two  auricles  and  two  ventricles)  thus  resem- 
bling the  condition  found  in  birds  and  mammals. 

The  only  crocodilians  in  the  United  States  are  the  alli- 
gator, Alligator  mississippiensis,  which  lives  in  the  rivers 
emptying  into  the  Gulf  of  Mexico,  and  the  American 
crocodile,  Crocodilus  americanus,  in  southern  Florida.  The 
alligator  may  attain  a  length  of  twelve  or  fourteen  feet. 
During  the  breeding  season  the  males  bellow  like  bulls 
and  give  off  a  penetrating  odor  from  two  musk  glands  in  the 
lower  jaw.  Nests  are  constructed  by  heaping  mounds  of 
rubbish  in  swampy  places  and  eggs  are  deposited  in  them. 
The  American  alligator  is  very  shy,  but  some  Indian  and 
African  crocodiles  attack  man. 

GENERAL   REMARKS   ON    REPTILIA 

Without  much  doubt  reptiles  arose  during  evolution  from 
aquatic  salamander-like  ancestors.  This  seems  probable 
both  from  the  palseontological  records  and  the  embryo- 
logical  changes  in  living  forms.  Fossil  remains  in  the 
stratified  rocks  show  that  the  first  vertebrates  on  earth 
were  fishes,  and  that  amphibians  preceded  reptiles.  A 
modern  reptile  during  its  embryology  is  at  first  fish-like, 
then  salamander-like,  and  in  both  stages  possesses  gill 
clefts  which  never  function -as  breathing  organs  but  are 
lost  before  hatching  takes  place. 

Modern  reptiles  are  for  the  most  part  truly 'terrestrial 
animals,  but  are  handicapped  to  some  extent  by  the  fact 
that  they  are  cold-blooded.  They  are  on  this  account  con- 
fined to  the  warmer  parts  of  the-  earth,  whereas^the  birds 
and  mammals  may  invade  the  frigid  regions.  Reptiles 
have  mastered  air-breathing  and  water  conservation,  so 


REPTILIA 


267 


that  they  may  live  on  land  without  danger  of  drying  up 
and  even  dwell  in  deserts,  but  have  never  attained  to  tem- 
perature regulation  like  the  "warm-blooded"  animals. 

During  past  ages,  however,  the  reptiles  were  a  mighty 
race,  and  at  one  time  ruled  the  earth.  They  were  special- 
ized along  diverse  lines  and  during  early  tertiary  times 
were  the  dominant  animals  'in  most  of  the  available  habi- 
tats (Fig.  101).  Icthosaurs,  plesiosaurs,  crocodiles,  and 
mososaurs  were  admirably  fitted  to  prey  upon  the  smaller 


FIG.  101. — Restorations  of  fossil  reptiles.  (Adapted  from  Knipe  and  Lucas.) 
A  pterodactyl  soars  through  the  air,  two  dinosaurs  are  walking  along  the  shore, 
and  a  mososaur  swims  in  the  water.  , 

animals  in  the  ocean,  before  there  were  whales  or  dolphins. 
The  empire  of  the  air  was  ruled  by  great  pterosaurs,  which 
were  somewhat  like  birds  in  form  but  possessed  teeth  and 
long  tails.  They  had,  like  birds,  many  adaptations  for 
aerial  life — such  as  wings  and  hollow  bones.  One  species 
could  spread  its  wings  about  twenty  feet.  The  land  was 
inhabited  by  giant  dinosaurs  and  other  reptiles,  some 
exceeding  a  hundred  feetT  in  length.  There  was  great 


268  GENERAL  ZOOLOGY 

diversity  in  habits  and  some  species  resembled  mammals 
in  structure.  There  were  carnivorous  dinosaurs  which 
looked  much  like  great  lizards  or  kangaroos;  and  herbivor- 
ous species  like  rhinoceri  and  buffaloes.  The  reptiles  of 
past  ages  occupied  the  places  now  taken  by  birds  and 
mammals  and  had  many  of  their  adaptations. 

But  the  great  abundance  of 'reptile  life  which  once  domi- 
nated the  earth  has  now  gone  forever.  Only  a  few  strag- 
glers remain  from  those  which  at  one  stage  in  the  earth's 
evolution  were  the  best  animals  that  had  been  produced. 
Nature  has  since  brought  forth  better  living  mechanisms. 
But  the  reptiles  did  not  live  in  vain,  for  evolutionists  tell 
us  that  some  of  the  extinct  forms  doubtless  gave  rise  to 
the  birds  and  mammals. 


CHAPTER  XXV 
SUBPHYLUM  VERTEBRATA,  CLASS  AVES 

Birds  differ  from  all  other  vertebrates  in  possessing 
feathers,  though  the  scaly  feet  suggest  relationship  to 
reptiles.  Certain  extinct  species  possessed  teeth  ,and  had 
separate  toes  ending  in  claws  on  the  front  limbs,  but  all 
modern  birds  have  the  toothless  jaws  covered  with  a  horny 
beak  and  the  digits  of  their  wings  are  more  or  less  rudi- 
mentary. Birds,  like  mammals,  are  "  warm-blooded  "- 
that  is,  are  capable  of  maintaining  a  constant  body 
temperature. 

There  are  two  subclasses  of  Aves: 

Subclass  1.  Archseronithes.^  Extinct  birds  having  toothed  jaws,  a 
long  lizard-like  tail  composed  of  separate  vertebrae,  and  three  separate, 
clawed  digits  on  each  of  the  fore  limbs. 

Subclass  2.  Neornithes.  Extinct  and  modern  birds  in  which  the 
terminal  vertebrae  of  the  tail  are  fused  to  form  a  pygostyle,  or  "plow- 
share" bone;  the  jaws  are  toothless,  except  in  some  extinct  species;  the 
bones  of  the  fore  limb  are  more  or  less  fused  and  reduced  in  size. 

SUBCLASS  1.    ARCHvEORNITHES 

This  group  of  birds  is  known  from  only  two  specimens 
and  a  separate  feather,  found  in  the  lithographic  slate  at 
Solenhofen,  Bavaria.  All  these  fossils  are  referable  to  a 
single  species,  Archceopteryxlithographica  (Fig.  102).  This 
bird  was  about  the  size  of  a  crow.  It  had  a  long  flexible 
tail  with  feathers  along  either  side,  and  the  wings  bore 
three  free  digits,  each  with  a  claw,  while  the  hind  legs  had 
four  toes,  like  those  of  modern  birds.  The  jaws  were 
armed  with  a  row  of  strong  teeth  set  in  sockets. 

Though  this  curious  flying  animal,  preserved  as  a  fossil 
for  our  inspection  from  Jurassic  times,  has  characteristics 

269 


270 


GENERAL  ZOOLOGY 


of  both  reptiles  and  birds,  as  judged  by  existing  representa- 
tives of  those  groups,  it  is  classed  witji  the  latter  and  is 
looked  upon  as  the  most  primitive  known  bird.  Archseop- 
teryx  had  feathers,  and  in  this  respect  differed  from  all 
reptiles,  living  or  fossil.  It  is  about  as  nearly  a  "  missing 
link"  as  science  may  reasonably  expect  to  find  when  the 
imperfection  of  the  fossil  record  is  considered,  and  its 


FIG.   102. — Archceopteryx  (A)   and  Hesperornis   (B),  two  extinct  toothed  birds. 
(After  restorations  by  Knipe  and  Lucas.) 

structure  is  believed  to  indicate,  with  other  evidence,  that 
birds  were  evolved  from  reptilian  ancestors. 


SUBCLASS  2.     NEORNITHES 

The  birds  in  this  group  are  characterized  by  having 
several  vertebrae  at  the  tip  of  the  tail  fused  to  form  a 
" plowshare  bone"  which  acts  as  a  support  for  the  large 
tail  feathers.  The  wing  bones  are  reduced  and  more  or 


AVES  271 

less  united;  there  are  never  more  than  two  free  digits  with 
claws,  and  usually  none.  Four  orders  of  fossil  birds  are 
placed  in  this  subclass,  as  are  seventeen  whose  representa- 
tives live  today.  Space  will  not  permit  the  discussion  of 
all  these;  they  will,  therefore,  as  a  matter  of  convenience 
be  divided  into  three  groups:  (1)  primitive  fossil  birds^ 
(2)  recent  flightless  birds;  (3)  recent  flying  birds.  2^  ' 

The  fossil  Neornithes  had  some  very  peculiar  features: 
Hesperornis  regalis  (Fig.  101)  was  a  flightless  bird  about 
four  feet  long,  adapted  for  swimming  and  diving.  It 
had  teeth  set  in  grooves  in  the  jaws;  the  sternum  was  with- 
out a  "keel"  for  the  attachment  of  wing  muscles,  and  the 
strong  hind  feet  were  webbed  for  swimming.  The  re- 
mains of  this  great  diver  have  been  found  in  the  Cretaceous 
deposits  in  Kansas.  In  the  same  region  the  remains  of 
other  aquatic  birds  which  had  keeled  sterna  and  jaws 
bearing  teeth  set  in  sockets  have  been  discovered. 

The  elephant-birds  (dZpyornis,  etc.)  and  the  moas 
(DiorniSj  etc.)  probably  became  extinct  within  the  past 
five  hundred  years.  The  former  were  great  flightless 
creatures  which  lived  in  Madagascar.  The  eggs  of 
^Epyornis  have  occasionally  been  dug  up  along  the  sea- 
shore; some  of  them  are  over  thirteen  inches  long  and  have 
a  capacity  of  a  couple  of  gallons.  Over  twenty  species  of 
moas  formerly  lived  in  New  Zealand.  They  were  unable 
to  fly  and  possessed  enormous  hind  limbs  adapted  for 
running.  Some  were  as  small  as  turkeys,  while  others 
stood  ten  feet  in  height.  Probably  the  early  human  in- 
habitants of  New  Zealand  exterminated  these  birds,  for 
remains  are  found  in  caves  and  in  ancient  refuse  heaps. 

Xmong  recent  birds  which  fly  but  little  or  not  at  all,  the 
kiwis  (Apteryx)  of  New  Zealand  are  the  most  striking. 
They  are  about  the  size  of  a  hen  and  have  a  very  long  beak^ 
which  they  use  as  a  probe  in  seeking  worms  underground. 
The  wings  are  very  degenerate,  being  represented  by  small 
bones  which  do  not  appear  outside  of  the  body;  tail  feathers 
are  wholly  lacking.  The  penguins  (ofder^Jjnpennes)  of 


272  GENERAL  ZOOLOGY 

which  about  twenty  species  are  known,  are  confined  to  the 
Antarctic  regions.  They  live  the  greater  part  of  the  time 
in  the  open  ocean  where  their  short  wings  (inadequate  for 
flight)  are  used  for  swimming.  When  on  land,  penguins, 
walk  erect  or,  if  in  haste,  slide  along  on  the  belly.  They 
nest  on  land  in  great  colonies  during  the  Antartic  summer 
at  temperatures  as  low  as  78°F.  below  zero.  Several  birds 
often  cooperate  in  incubating  the  eggs  and  caring  for  the 
young. y^The  cassowaries,  emfcus,  ostriches,  and  rheas'  are 
large-inning  birds  with  small  wings  and  large,  powerful 
legs.  The.  ostrichesDop  camol  birds  (Struthio -eamvlus) , 
live  in  the  desert  regions  e£^AMea,  and  travel  about  in 
groups.  These  are  the  largest  living  birds,  some  indi- 
viduals attaining  a  height  of  eight  feet  and  weighing  over 
three  hundred  pounds.  They  are  very  shy  and  flee  swiftly 
when  approached;  but  do  not  hide  their  heads  in  the  sand, 
as  is  commonly  believed.  Ostrich  farming  is  now  a  well- 
established  industry  in  South  Africa  and  certain  parts  of 
the  United  States.  A  domesticated  ostrich  will  yield 
fifteen  or  twenty  dollars  worth  of  plumes  each  year.  The 
utilization  of  such  feathers  is  desirable,  for  plucking  causes 
the  ostrich  little  inconvenience  and  the  use  of  the  plumes 
saves  the  lives  of  many  wild  birds  which  might  otherwise 
be  slaughtered  on  the  altar  of  fashion.  The  tinamous  are 
birds  inhabiting  Mexico,  Central  and  South  America. 
They  resemble  partridges  in  appearance,  but  fly  very  little. 
The  flying  birds  are  divided  intovteHr  orders.  The  loons 
and  grebes  (Colymbiformes)  are  strong  flyers  but  are  chiefly 
remarkable  "for  their  ability  to  swim  and  dive.  A  loon  can 
stay  under  wat&\several  minutes,  and  travel  a  quarter  of 
a  mile  while  submerged./  The  albatrosses,  fulmars,  shear- 
waters, and  petrels  (P^ocellariformes)  are  gull-like  marine 
birds  with  tubular  nostrils\and  long  slender  wings.  They 
are  expedient  flyers  and  riesVin  grWt  colonies  on  Oceanic 
islands. /The  stork-like  birds  (Ciconiiformes)  include  the 
tropic  birds,  Ciinnoran-ts,  anhingas,  pelicans/  herons/  bit- 
terns, spoonbills,  storks,  ibises,  flamingoes/etc.  Most  of 


AVES  273 

these  birds  have  long  legs,  slender  necks,  elongated  bills, 
and  feet  fitted  for  wading  or  swimming.  I'he  geese,  river- 
ducks,  sea-ducks,  fishr ducks,  swans,  and  screamers  (Anseri- 
formes)  have  webbed  feet  and  are  aquatic  in  their  habits. 
Hawks,  eagles,  vultures  an4  secretary  birds  (Falconi- 
formes)  are  the  true  birds  of  prey.  The  owls,  which  in  the 
popular  mind  are  associated  with  them — probably  as  much 
because  of  their  food  habits  as  for  any  other  reason — belong 
to  a  different  order.  All  the  birds  of  prey  have  very  strong 
talons  and  hooked  beaks,  which  are  used  for  catching  and 
killing  animals  for  food.  The  owls,  together  with  the 
kingfishers,  hummingbirds,  goatsuckers,  woodpeckers,  and 
a  few  other  rare  forms  are  placed  together  in  the  order 
Coradiformes  because  of  certain  anatomical  similarities. 
Owls  are  generally  nocturnal,  but  are  able  to  see  more  or 
less  during  the  day.  Their  great  expanse  of  wing,  together 
with  a  peculiar  frilling  of  the  feathers,  enable  them  to. fly 
with  a  quietness  remarkable  in  such  large  birds.  The 
hummingbirds  are  the  smallest  of  the  class  Aves,  the  largest 
being  less  than  eight  inches  long,  and  the  smallest,  two  and 
three-eighths  inches.  Over  four  hundred  species  are  known 
in  America,  but  only  one  .is  found  throughout  United  States. 
They  are  nearly  all  of  gorgeous  metallic  colors,  changing 
in  different  lights.  The  woodpeckers  are  remarkable  for 
their  peculiar  beaks,  extensible  tongues;  the  stiff  tail  and 
peculiarly  arranged  toes  both  function  in  climbing. ,.  The 
rails,  cranes,  and  coots  (Gruiformes)  are  marsh  hkds_wjth 
long  legs  for  wading  or  with  lobate  feet  for  running  over 
aquatic  -vegetation.  Snipes,  plovers,  curlews,  and  jacanas 
are  related'to  the  gulls,  terns,  auks,  and  pigeons  (Charadrii- 
formes).  The  snipes  and  their  near  relatives  are  waders; 
the  gulls  are  strong  flyers  and  live  largely  on  fish ;  the  auks 
are  sea  bi^ds  with  very  heavy  beaks,  and  nest  in  holes  in 
the  ground;  the  gentle  pigeons  and  doves  are  widely  dis- 
tributed, three  hundred  species  being  known.  The  cuckoo- 
like  birds  (Cuculi/ormes)  include  the  true  cuckoos,  parrots, 
and  a  number  of  tropical  birds.  The  cuckoos  are  insect 


274  GENERAL  ZOOLOGY 

eaters,  and  are  chiefly  known  from  the  -habits  of  the 
European  species,  which  deposits  its  eggs  in  the  nests  of 
other  birds.  Parrots  live  largely  on  fruits  and  seeds. 
Only  one  rare  species,  the  Carolina  Paroquet,  occurs  in  the 
United  States,  and  it  is  altogether  probable  that  this  has 
become  extinct  during  the  last  few  years. 

Most  of  the  familiar  small  birds  belong  to  the  order 
Passeriformes,  which  includes  almost  half  {7000)  of  the 
known  species  of  birds.  There  are  sixty-feur  families  in 
this  important  order — the  -finches,  flycatchers,  vireos, 
thrushes,  wrens,  blackbirds,  jays,  swallows,  warblers,  and 
many  others  being  included.  Passerine  birds  are  usually 
of  small  or  medium  size,  but  are  the  most  highly  organized 
of  the  class  Aves.  The  feet  are  fitted  for  perching,  and 
representatives  of  the  otfder  are  often  called  "  perching 
birds."  A  familiar  example  is  the  American  robin,  which 
will  be  considered  in  some'  detail.  This  bird  is  not  closely 
related  to  the  European  " robin  red-breast,"  which  belongs 
in-an  entirely  different  family.  It  is  familiar  to  everyone 
iiu  North  America  because  of  its  trusting  ways  and  the 
readiness  with  which  it  adjusts  itself  to  the  changes  accom- 
panying the  advance  of  civilization. 

THE  AMERICAN  ROBIN,  Planesticus  migrat&rius  Linnaeus 

Self^-maintenance. — The  robin  eats  caterpillars,  insects, 
earthworms,  and  other  small  animals.  Everyone  has  seen 
it  straining  and  tugging  to  pull  a  fat  worm  from  the  lawn. 
There  has  been  some  dispute  among  students  of  bird  foods 
as  to  whether  the  robin  is  beneficial  or  injurious  to  man. 
Barrows  says:  " There  is  no  question  that  the  robin  some- 
times does  a  large  amount  of  good  in  its  consumption  of 
insects,  especially  by  eating  cutworms  and  grasshoppers; 
it  must  be  remembered,  however,  that  the  major  part  of 
its  insect  food  is  taken  from  the  ground  and  that  hence  the 
robin  is  a  factor  of  small  importance  in  limiting  the  activity 
of  the  spanworms  and  other  caterpillars  which  defoliate 
our  fruit  and  shade  trees.  It  also  eats  large  numbers  of 


AVES  275 

insects  which  at  best  are  not  harmful,  and  which  possibly 
may  be  beneficial."  Forbes  states  that,  " while  the  robin 
is  not  so  precious  that  we  need  make  it  an  act  of  sacrilege 
to  show  him  the  muzzle  of  a  gun  in  a  cherry  orchard — on 
the  other  hand  it  would  be  an  enormous  blunder  to  wage 
ourselves,  or  to  permit  others  to  wage,  any  general  or  indis- 
criminate war  against  him." 

The  robin  depends  for  the  most  part  on  its  acute  vision 
and  agility  to  secure  food.  Its  sharp  eyes  are  quick  to  spy 
out  any  lurking  insect  in  a  garden,  and  its  horny  beak  is  an 
excellent  organ  for  capturing  such  morsels,  for  it  is  im- 
pervious to  bites  or  stings.  Birds  are  handicapped  some- 
what in  feeding  by  the  lack  of  claws  or  other  seizing  organs 
on  the  front  limbs ;  but  this  is  compensated  for  by  the  great 
quickness  which  is  associated  with  the  use  of  the  wings  for 
flight,  and  by  the  great  flexibility  of  the  neck. 

The  feathers  are  admirably  adapted  for  flight;  being 
light,  yet  with  broad  expanse;  flexible,  but  stiff  enough  to 
resist  the  air.  The  small  barbs  on  either  side  of  the  main 
shaft  are  provided  with  minute  hooked  barbels  which  bind 
them  together  so  as  to  make  a  broad  flat  surface;  this  may 
be  broken  up  repeatedly,  yet  on  being  smoothed  over  again 
will  be  as  firm  as  before.  Within  the  body  there  are  struc- 
tural adaptations  which  make  it  light.  The  air  sacs  open- 
ing from  the  lungs  are  good  examples  of  such  structures. 
They  lie  beween  the  large  muscles,  among  the  internal 
organs,  and  are  even  connected  with  cavities  within  the 
long  bones.  In  order  that  flight  may  be  swift  and  sure, 
the  body  must  be  rigid  as  well  as  light;  there  can  be  no 
wavering  or  the  flyer  will  fall.  To  secure  greater  rigidity 
in  the  body,  certain  parts  of  the  skeleton  which  consist  of 
separate  bones  in  other  vertebrates  are  firmly  united  in 
birds.  Each  rib  bears  a  projection  (uncinate  process)  on 
one  side  which  overlaps  the  next  rib,  thus  making  the  body 
very  firm  during  flight.  Perhaps  the  most  striking  skeletal 
adaptation  for  flight  is  the  great  keel  on  the  sternum,  which 
gives  attachment  to  the  wing  muscles. 


276  GENERAL  ZOOLOGY 

The  robin  is  not  only  well  equipped  to  find  and  capture 
food,  but  is  also  able  to  profit  by  experience  to  a  greater 
degree  than  a  toad  or  a  lizard.  It  soon  learns  to  avoid  un- 
desirable objects,  and  usually  hunts  in  favorable  places 
at  the  most  opportune  times.  After  the  food  has  been 
captured  and  swallowed,  it  is  stored  in  the  crop,  ground 
among  the  pebbles  which  are  present  in  the  muscular 
gizzard,  and  then  absorbed  in  the  intestine.  Digestion, 
absorption,  assimilation,  and  excretion,  are  assisted  by  the 
digestive  glands  (liver,  pancreas)  and  by  certain  ductless 
glands  (pancreas,  thyroid,  thymus)  which  act  " indirectly" 
by  secreting  substances  into  the  blood. 

The  circulatory  system  of  birds  presents  some  points  of 
special  interest.  At  one  time  within  the  egg  a  robin  has 
several  pairs  of  aortic  arches,  but  these  degenerate  or  are 
diverted  to  various  parts  of  the  body  as  development  pro- 
ceeds, and  an  adult  bird  has  only  one  great  aortic  arch, 
which  curves  to  the  right  after  leaving  the  heart  and  then 
runs  backward  as  the  dorsal  aorta  (Fig.  105).  The  heart 
in  all  birds  has  four  chambers — the  left  auricle  and  ven- 
tricle collect  and  pump  blood  to  the  lungs  for  aeration; 
the  right  side  receives  the  "pure"  blood  from  the  lungs 
and  forces  it  through  the  body.  Because  of  the  active  life 
a  bird  leads,  metabolism  is  very  rapid.  The  body  tem- 
perature is  even  higher  than  that  of  mammals,  and  respira- 
tion is  accordingly  accentuated.  A  bird  breathes  through 
its  lungs  into  air  sacs  beyond. 

The  excretory  products  of  a  robin  pass  into  the  cloaca 
from  the  kidneys  and  are  eliminated  through  the  anus 
with  the  faeces.  They  are  chiefly  in  the  form  of  uric  acid, 
which  is  a  solid  and  therefore  well  adapted  to  an  aerial 
animal.  A  robin  must  be  light  and  could  not  carry  a 
great  quantity  of  water,  which  would  be  necessary  to 
dissolve  urea. 

Self-protection. — A  robin's  keen  eyes  and  ears  usually 
give  adequate  notice  of  approaching  danger;  any  shadow 
which  indicates  a  prowling  hawk  causes  a  speedy  retreat 


AVES  277 

to  some  thicket;  any  unusual  sound  causes  the  sharp 
eyes  to  seek  its  source.  Raptorial  birds,  weasels,  snakes 
and  other  animals  catch  robins  when  they  can,  and  are 
particularly  likely  to  prey  upon  young  individuals.  In  a 
general  way  the  coloration  of  a  robin  protects  it  from  such 
sharp-eyed  enemies.  The  body  is  countershaded  (dark 
above  and  light  below)  so  that  it  is  not  conspicuous  when 
illuminated  from  above,  because  the  lighter  parts  are  in 
the  deepest  shadow.  The  pigments  in  the  feathers  also 
blend  well  with  the  tree  trunks  and  branches  which  robins 
usually  frequent.  When  a  robin  is  alarmed  or  threatened 
with  danger  it  gives  certain  call  -notes  which  warn  its 
fellows  of  the  trouble,  so  that  they  may  either  look  out 
for  themselves  or  come  to  investigate  the  cause  of  the 
disturbance. 

A  robin  is  much  better  equipped  than  a  lizard  to  meet 
the  varying  conditions  of  a  terrestrial  environment.  Both 
have  a  dry  skin  which  conserves  the  water  within  the 
body,  but  feathers  are  better  than  scales  because  they 
keep  a  layer  of  air  next  to  the  body  which  serves  as  an 
insulator  against  loss  of  moisture  by  evaporation  or  of  heat 
by  radiation.  A  bird  is  also  more  specialized  than  a  lizard 
in  having  a  constant  body  temperature  which  is  main- 
tained in  spite  of  biting  cold  or  extreme  heat.  Most  robins 
migrate  southward  to  pass  the  winter  in  a  warm  climate 
where  food  is  abundant,  but  a  few  may  linger  in  certain 
northern  localities  until  spring.  The  migration  of  the 
robin  covers  a  distance  of  approximately  3000  miles.  In 
autumn  the  trip  takes  about  eighty  days,  and  in  spring, 
seventy.  The  birds  move  northward  as  the  mean  daily 
temperature  reaches  35°F. 

Birds  are  subject  to  attack  by  various  diseases.  A  robin 
may  acquire  bird  malaria  from  the  bites  of  mosquitoes, 
and  certain  bacteria  may  cause  internal  disorders.  Trema- 
todes  and  other  worms  live  within  the  body;  bird  lice  and 
other  ectoparasites  dwell  among  the  feathers.  Such  dis- 
eases and  parasites  the  robin  avoids  as  far  as  possible  by 


278  GENERAL  ZOOLOGY 

scrupulous  cleanliness.  Frequent  baths  are  taken,  both 
in  water  and  dust  (the  latter  being  distasteful  to  ectopara- 
sites), the  feathers  are  preened  with  the  beak,  and  every 
precaution  is  taken  to  keep  the  body  in  good  condition. 

Race  Preservation. — Though  the  female  robin  shoulders 
the  chief  responsibilities  for  the  care  of  the  eggs  and  the 
rearing  of  the  young,  the  male  is  always  at  hand  to  en- 
courage his  mate,  sound  the  alarm  when  danger  threatens, 
or  attack  intruders.  The  pair  select  a  suitable  site,  usually 
in  an  elm,  maple,  or  apple  because  these  trees  have  broad 
forks  in  the  branches,  and  the  female  builds  the  nest.  She 
first  lays  down  a  rough  foundation  of  coarse  stems,  then 
brings  load  after  load  of  grass  and  mud  in  her  beak  and 
builds  a  wall  around  the  edge.  Each  time  new  material 
is  added  she  scratches  it  into  place  with  her  feet,  smooths 
and  shapes  it  with  her  breast,  and  rounds  off  loose  strands 
on  the  outside  with  her  beak.  The  moulding,  turning  and 
smoothing  movements  are  repeated  hundreds  of  times. 
Finally  a  lining  of  soft  grass  or  roots  is  placed  in  the  com- 
pleted cup  and  the  nest  is  ready  for  the  eggs.  The  male  is 
constantly  at  hand  and  seems  to  furnish  incentive  to  the 
female  to  carry  on  the  work.  If  she  goes  for  mud  he  fol- 
lows a  few  feet  behind,  singing  and  showing  great  interest 
in  all  she  does.  If  some  other  bird  or  small  animal  ap- 
proaches the  nesting  place,  the  male  attacks  the  intruder 
with  great  ferocity;  if  a  danger  threatens  that  cannot  be 
combated  he  gives  loud  warning  notes  and  stays  near  his 
mate  to  render  assistance. 

It  requires  from  two  to  four  days  to  complete  a  nest  and 
the  first  egg  is  usually  deposited  from  the  fourth  to  the 
seventh  day.  After  three  or  four  eggs  have  been  laid 
the  female  begins  to  incubate  them,  and  for  nearly  two 
weeks  leaves  the  nest  only  for  short  intervals  to  snatch  a 
little  food.  The  male  is  always  near  at  hand,  and  helps  in 
feeding  the  young  birds  after  they  hatch,  but  his  mate 
still  does  the  greater  part  of  the  work.  The  young  have 
ravenous  appetites,  eating  more  than  their  own  weight 


AVES 


279 


every  day.  The  attentive  parents  have  to  work  from 
"early  morn  till  dewy  eve"  to  keep  them  satisfied.  The 
nest  is  kept  perfectly  clean,  all  excrement  or  other  dirt 


(Photos  by  A   R.  Cahn.) 

FIG.  103. — "The  wind  blows  east,  the  wind  blows  west, 
The  blue  egg  in  the  robin's  nest 
Will  soon  have  beak  and  wings  and  breast, 
And  flutter  and  fly  away." 

being  carried  away  by  the  parents.  Even  after  the  young 
have  left  the  nest  and  are  making  short  flights  or  hiding 
in  the  bushes,  the  parents  watch  over  them. 


280  GENERAL  ZOOLOGY 

The  long  period  of  care-free  youth  which  robins  enjoy 
enables  them  to  reach  a  state  of  comparative  independence 
under  the  protection  of  the  parents.  They  do  not  en- 
counter the  force  of  the  struggle  for  existence  until  they 
have  attained  somewhat  of  strength,  vigor,  and  experience. 
The  egg  cell  is  stored  with  yolk,  fertilized,  and  has  passed 
through  early  segmentation  stages  before  it  is  laid.  As  it 
passes  down  the  oviduct  more  nourishment  is  added  around 
the  egg  proper  in  the  form  of  albumen,  or  "  white,"  and  the 
protective  shell  is  then  formed  on  the  outside.  While  the 
young  robin  develops  within  the  egg,  it  is  constantly 
warmed  and  watched  by  the  parents.  After  hatching, 
it  is  long  the  object  of  solicitous  attention.  Small  wonder, 
then,  that  most  robin  eggs  come  to  maturity. 

A  perch  lays  two  thousand  eggs,  gives  no  attention  to 
its  young,  and  most  of  them  are  destroyed.  A  robin  lays 
four  eggs  and  usually  brings  all  of  them  to  maturity.  Why 
is  one  method  better  than  the  other?  The  net  result  in 
either  case  will  perhaps  be  two  or  three  new  animals  from 
each  pair  annually.  The  robin,  however,  has  one  ad- 
vantage— the  young  pass  through  a  long  adolescent  period 
during  which  they  are  cared  for,  and  even  trained  to  some 
extent  by  having  opportunity  to  imitate  their  elders. 
The  bird  has  more  chance  to  acquire  experience  before 
being  thrown  on  its  own  resources,  and  this  gives  opportu- 
nity for  a  higher  grade  of  psychic  development. 

GENERAL  REMARKS  ON  BIRDS 

Birds  all  agree  in  having  feathers,  beaks,  arid  .character- 
istic feet,  but  all  these  structures  show  endless  variations 
which  are  usually  correlated  with  differences  in  habits  and 
habitats.  For  example  the  feathers  may  be  downy,  or 
stiff  and  hair-like;  the  feet  may  be  webbedj  feathered,  or 
naked;  the  beak  may  be  hooked  for  tearing  flesh,  short  and 
heavy  for  cracking  seeds,  or  long  and  slender  for  probing 
in  the  mud.  Each  type  of  bird  has  the  bodily  parts  highly 


AVES  281 

adapted  for  a  particular  mode  of  existence,  which  shows 
that  birds  are  racially  specialized. 

Birds  are  unusual  when  compared  with  other  vertebrates 
in  having  the  ability  to  fly.  Their  exceptional  agility  has 
made  it  easy  for  them  to  capture  food  and  escape  from 
enemies,  but  they  are  not  on  the  whole  as  versatile  as 
mammals,  partly  because  they  are  too  specialized.  They 
depend  largely  upon  quickness  and  the  acuteness  o?  their 
sense  organs.  This  has  led  to  the  extreme  development  of 
those  parts  of  the  nervous  system  which  correlate  accurate 
muscular  movements  and  control  reflexes;  but  the  parts 
which  have  to  do  with  thinking  and  scheming  are  com- 
paratively simple.  In  the  brain  of  a  bird  the  cerebellum 
(which  is  concerned  largely  with  the  coordination  of 
muscular  activities)  is  very  large  and  the  cerebral  lobes 
(where  higher  mental  qualities  reside)  are  small.  In  a 
mammal  both  regions  are  well  developed.  Birds,  then, 
as  a  race,  have  sacrificed  their  power  to  develop  great 
mental  ability,  probably  because  they  became  able  to  fly 
early  in  their  evolution,  and  then  were  so  specialized  that 
they  could  not  branch  off  on  new  evolutionary  lines.  The 
toes  on  the  front  limbs  became  degenerate  to  allow  the 
formation  of  more  effective  organs  for  flight;  the  neck 
grew  long  and  flexible  to  compensate  for  the  resulting  handi- 
cap in  securing  food;  the  teeth  gave  place  to  a  horny  beak. 
Specialization  along  such  lines  for  a  time  made  birds  so 
successful  that  they  became  modified  structurally  to  such 
an  extent  that  they  can  never  be  racially  youthful  and 
have  broad  possibilities  again.  Birds  as  a  race  are  in  their 
old  age. 

As  long  as  birds  succeed' with  their  present  adaptations, 
however,  they  will  dominate  the  air,  and  must  be  given 
credit  for  their  exceptional  flying  ability.  A  fish-hawk 
can  move  as  fast  as  an  express  train  and  may  feed  at  will 
from  the  ocean  or  an  inland  lake,  on  the  top  of  a  mountain 
or  in  the  bottom  of  a  canyon.  The  best  flyers  among 
birds  are  the  long-winged  gulls,  vultures,  hawks,  and  man- 


282 


GENERAL  ZOOLOGY 


Breeding 

Wintering 

Principal  migration  routes 


FIG.  104. — Distribution  and  migration  of    the  Eskimo  curlew.     (From  Cooke; 
Yearbook,  U.  S.  Department  of  Agriculture,  1914.) 


AVES  283 

V 

o'-war  birds.  Though  most  short-winded  birds  are  poor 
flyers  and  tire  easily,  some  sturdy  forms  ;  like  the  ducks 
(which  may  attain  a  speed  of  a  hundred  miles  per  hour) 
are  swift  and  effective  on  the  wing. 

The  seasonal  migration  of  birds  has  been  a  subject  of 
much  interest  and  speculation,  but  many  of  its  aspects  are 
still  shrouded  in  mystery.  Most  small  birds  move  about 
twenty-five  miles  a  day  on  their  journeys  north  and  south. 
They  usually  travel  high  in  air  at  night  and  rest  during  the 
day  in  appropriate  localities.  The  Arctic  tern  annually 
travels  "from  pole  to  pole,"  thus  living  in  perennial 
summer.  The  Eskimo  curlew  (Fig.  104)  covers  a  great 
ellipse — south  across  2500  miles  of  Atlantic  ocean  to  its 
winter  home  and  north  through  the  center  of  North 
America  to  its  breeding  grounds  within  the  Arctic  Circle. 
On  the  other  hand  some  birds,  like  the  quail  and  the  English 
sparrow,  migrate  little  or  not  at  all.  Migratory  birds 
usually  breed  in  the  coolest  part  of  their  range  and  spend  the 
summer  in  warmer  climates.  The  abundance  of  food  and 
the  presence  of  appropriate  nesting  sites  are  probably 
important  factors  in  controlling  such  flights  but  they  are 
not  the  sole  causes  of  migration;  in  some  species,  in  fact, 
appear  to  have  little  or  no  effect. 

The  songs  of  birds  commonly  serve  for  the  attraction 
of  mates,  but  various  other  characteristic  sounds  are  useful 
for  warning  signals,  calling  the  young,  etc.  The  vocal 
apparatus  of  a  bird  is  not  in  the  larynx,  as  in  mammals, 
but  lies  in  the  "syrinx"  at  the  lower  end  of  the  trachea  and 
is  even  sometimes  imbedded  in  the  .sternum.  It  is  a  rather 
complicated  apparatus,  capable  in  many  cases  of  producing 
a  considerable  range  of  sounds. 

The  colors  of  birds  are  for  the  most  part  protective. 
The  body  is  countershaded  (i.e.,  is  dark  above  and  light  be- 
low, so  that  the  effects  of  light  and  shadow  are  eliminated,) 
and  is  therefore  inconspicuous.  Some  birds,  like  the  grouse, 
also  have  accurate  pictures  on  the  feathers  of  the  back- 
grounds on  which  they  are  most  apt  to  be  seen.  The  fact 


284  GENERAL  ZOOLOGY 

that  a  bird  is  bright  colored  is  not  necessarily  to  be  taken 
as  indicating  that  it  is  conspicuous  in  its  usual  habitat. 
Parrots,  for  example,  are  very  difficult  to  see  among  the 
foliage  of  tropical  trees.  In  some  instances,  however, 
striking  colors,  may  serve  "for  attracting  the  opposite  sex, 
or  as  recognition  marks  for  other  members  of  the  same 
species. 

The  nesting  habits  of  birds  show  various  degrees  of  com- 
plexity and  specialization  in  styles  of  architecture.  A 
nighthawk  builds  no  nest,  but  deposits  the  eggs  on  the 
bare  ground,  or  sometimes  even  on  a  gravel  roof.  The 
burrowing  owl  of  the  western  prairies  digs  a  hole  and 
strews  manure  on  the  bottom  to  keep  the  eggs  off  the 
ground.  The  bush-turkey  of  Australia  builds  great  mounds 
of  sticks  and  dead  leaves  in  which  the  eggs  are  left  to  incu- 
bate from  the  heat  generated  by  the  decaying  rubbish. 
Most  passerine  birds  build  nests  like  the  robin,  on  the 
ground  or  among  the  branches  of  plants.  The  apex  of 
avian  architecture  is  reached  in  the  beautiful  basket  nests 
of  the  orioles. 

With  a  few  exceptions  birds  incubate  the  eggs  in  nests  or 
cavities  which  they  construct.  In  general  the  length  of 
the  incubation  period  is  proportional  to  the  size  of  the 
egg,  the  smallest  hatching  most  quickly.  The  emperor 
penguin  carries  its  single  egg  between  the  hind  legs,  and 
male  and  female  birds  take  turns  holding  it.  Eggs  laid 
in  open  nests  are  usually  colored  to  match  the  surround- 
ings, and  this  is  particularly  striking  in  those  which  are 
laid  on  the  ground  without  a  nest  or  with  but  a  very  crude 
one,  as  in  the  nighthawk  or  killdeer.  Birds  which  nest 
in  hollow  trees  or  in  holes  in  the  ground  usually  lay  white 
eggs. 

Domesticated  birds  play  a  considerable  part  in  modern 
civilization.  The  numerous  varieties  of  the  common  hen 
probably  all  come  originally  from  the  jungle-fowl,  Gallus 
gallus,  of  India.  Domestic  pigeons  were  derived  from  the 
blue-rock  pigeon,  Columba  livia,  a  native  of  Europe  and 


AVES  .  285 

Eastern  Asia.  Most  of  our  tame  geese  originated  from  the 
graylag  goose  of  northern  Europe,  and  our  ducks  from  the 
mallard.  The  peacock  is  a  native  of  India,  and  the  Guinea 
fowl  came  originally  from  West  Africa.  The  turkey  has 
been  domesticated  since  the  white  man  came  to  America, 
and,  though  rapidly  becoming  scarce,  is  still  found  in  a 
wild  state. 


•     CHAPTER  XXVI 

SUBPHYLUM  VERTEBRATA,  CLASS 
MAMMALIA 

A  mammal  is  a  warm-blooded  vertebrate  with  hair  and 
mammary  glands.  There  are  a  few  apparent  exceptions  to 
this  definition.  For  example,  some  whales  as  adults  are 
naked,  in  one  instance  having  the  hair  reduced  to  two  bris- 
tles on  the  upper  lip;  but  before  birth  all  have  an  abundant 
hairy  covering.  Mammary  glands  are  found  in  all  mam- 
mals. These  are  simply  specialized  skin  glands  which 
secrete  milk  to  nourish  the  young.  The  ancestors  of 
mammals  were  probably  for  the  most  part  land  animals, 
but  in  recent  geological  history  many  have  become  adapted 
to  live  in  the  water.  Even  the  whales  appear  to  have  de- 
scended from  quadrupedal  forms  which  lived  along  the 
shores  of  ancient  oceans. 

All  mammals  have  four-chambered  hearts,  like  birds  and 
crocodiles.  They  also  have  a  single  aortic  arch  carrying 
blood  from  the  heart  to  the  dorsal  aorta,  but  in  this  case  the 
one  on  the  left  side  has  persisted  instead  of  that  on  the 
right  as  in  birds.  In  passing  from  fishes  to  mammals  the 
aortic  arches,  which  in  the  simplest  vertebrates  carry  blood 
to  and  from  the  gills,  are  progressively  lost  or  diverted  to 
other  uses;  the  heart  on  the  contrary  becomes  more  and 
more  complicated  (Fig.  105).  Fishes  (A)  usually  have 
four  pairs  of  functional  arches  which  supply  the  gills,  and 
possess  a  tubular  heart;  adult  amphibians  (B)  have  three 
pairs  of  aortic  arches  (only  one  of  which  connects  with  the 
dorsal  aorta;  the  other  two  being  diverted  to  the  head,  lungs 
and  skin) ;  the  reptiles  (C)  have  arches  somewhat  like  am- 
phibians, but  in  the  crocodiles  acquire  a  four-chambered 
heart;  in  the  birds  (D)  and  mammals  (E)  only  one  arch  of  a 

286 


MAMMALIA 


287 


single  pair  connects  with  the  dorsal  aorta  and  the  heart  is 
always  four-chambered. 

The  teeth  of  mammals  show  great  variations  which  in 
general  are  correlated  with  differences  in  feeding.  The 
reptilian  ancestors  of  the  mammals  possessed  teeth  and  all 
existing  mammals  have  them  at  some  time.  The  curious 
duck-bill,  certain  whales,  armadillos  and  the  ant  eaters 
have  very  rudimentary  teeth,  however,  which  do  not  break 
through  the  jaw  and  are  resorbed  before  birth.  Most 
mammals  have  two  sets  of  teeth — a  temporary  or  "milk" 
dentition  and  a  permanent  dentition.  In  some  (guinea 


FIG.   105. — A  comparison  of  the  heart  (dotted)   and  chief  arteries  in:  A,  fish; 
B,  frog;  C,  lizard;  D,  bird;  E,  mammal. 

pigs,  bats)  the  former  is  lost  before  birth.  The  teeth  may 
be  all  alike  (homodont  dentition),  as  in  the  dolphins,  or 
show  a  differentiation  into  incisors,  canines,  premolars,  and 
molars  (heterodont  dentition).  The  molars  appear  only 
in  the  permanent  dentition.  Very  often  animals  which 
have  a  specialized  heterodont  dentition  may  lack  certain 
types  of  teeth,  and  a  diastema,  or  bare  space,  is  left  in  the 
jaw.  A  squirrel,  for  example,  has  very  large  incisors  but 
no  canines  .and  there  is  a  gap  in  front  of  the  premolars;  a 
cow  lacks  incisors  in  the  upper  jaw  and  has  no  canines 
whatever. 


288 


GENP:RAL  ZOOLOGY 


In  the  mammals  structures  for  reproduction  and  the 
nourishment  of  the  young  have  attained  far  greater  special- 
ization than  in  any  other  group  of  vertebrates.  The  sexes 
are  always  separate,  the  egg  is  fertilized  within  the  female 
and  early  stages  of  development  take  place  within  her  body. 
The  most  primitive  mammals  lay  soft-shelled  eggs  some- 
what like  those  of  birds,  but  the  majority  have  the  egg 


body  wall 


FIG.   106. — Section  through  body  wall  and  uterus  of  a  placentate  mammal  to 
show  how  the  embryo  is  attached  to  the  wall  of  the  uterus. 

develop  within  the  body  and  the  young  are  nourished  by 
the  mother  for  some  time — in  many  cases,  both  before  and 
after  birth.  The  young  of  the  marsupials  are  at  first  very 
helpless.  They  are  placed  at  birth  in  the  marsupial 
pouch  on  the  ventral  side  of  the  abdomen,  where  they  re- 
ceive protection  and  are  nourished  from  the  mammary 
glands.  All  mammals  which  do  not  lay  eggs  or  place  the 
young  in  a  marsupial  pouch  have  a  placenta  (Fig.  106)  of 
some  sort.  This  is  a  complicated  structure  which  grows 


MAMMALIA  289 


out  from  the  developing  embryo  and  becomes  fastened  to 
the  uterus  of  the  mother  in  such  a  way  that  there  is  an 
exchange  of  nourishment  between  mother  and  offspring 
through  the  thin  walls  of  the  blood-vessels.  All  mamma- 
lian embryos,  like  those  of  reptiles  and  birds,  are  covered  by 
a  protective  envelope,  the  amnion,  before  birth.  This  is 
filled  with  a  watery  fluid  and  its  walls  enter  into  the  forma- 
tion of  the  placenta. 

Mammals  are  divided  into  large  groups  primarily  on  the 
basis  of  the  degree  of  specialization  in  the  reproductive  and 
developmental  processes.  The  groups  are  as  follows : 

Subclass  I.     Prototheria;  egg-laying  (oviparous)  mammals. 

Subclass  II.     Eutheria;  viviparous  mammals. 

Division  1.  Didelphia;  mammals  which  carry  the  young  in  a  mar- 
supial pouch  and  nourish  them  before  birth  through  a  placenta  which  is 
usually  very  primitive. 

Division  2.  Monodelphia;  mammals  which  nourish  the  young 
through  a  typical  placenta,  and  never  carry  them  in  a  pouch  after  birth. 

Fossil  records  show  that  the  Prototheria  and  Didelphia 
appeared  on  earth  before  the  Monodelphia.  There  are 
also  rudimentary  structures  on  various  representatives  of 
the  latter  group  which  indicate  that  they  came  from 
marsupial  ancestors.  The  fourteen  orders  of  mammals 
which  have  living  representatives  will  now  be  considered. 

Order  1.  Monotremata. — This  order  includes  two  primi- 
tive egg-laying  mammals  which  live  in  Australia,  New  Gui- 
nea, and  Tasmania.  The  skeleton  and  some  other  mor- 
phological features  show  affinities  with  birds  and  reptiles. 
The  eggs  hatch  in  a  few  hours  and  the  young  are  at  once 
placed  in  a  hollow  in  the  abdomen  which  is  lined  with  mam- 
mary glands.  There  is  no  teat  and  the  young  lick  the 
milk  from  the  hairy  surface  of  the  skin. 

The  spiny  ant  eater,  Echidna  aculeata,  has  a  long  head 
and  a  mouth  without  teeth.  Its  tongue  is  very  sticky  and 
extensile,  serving  to  capture  ants.  The  duckbill,  Orni- 
thorhynchus  anatinus  (Fig.  107)  is  adapted  to  an  aquatic 
life.  It  possesses  webbed  feet,  thick  fur,  and  a  duck-like 


290 


GENERAL  ZOOLOGY 


beak  with  which  it  catches  worms  and  insects  under  water. 
The  male  has  poisonous  spurs  on  the  heels  of  his  hind  legs. 
/Order  2.  Marsupialia. — The  marsupials  occur  chiefly  in 
the  region  of  Australia,  where  they  show  considerable  diver- 
sity. The  dasyures  and  Tasmanian  devil  are  carnivorous; 
the  kangaroos  (-Fig.  107)  and  wallabys  are  vegetarians;  the 
pouched  moles  burrow  underground  for  insects;  the  pha- 
langers  live  in  trees  and  have  prehensile  tails.  Yet,  despite 
such  variations  in  habits,  with  corresponding  structural 
adaptations,  all  marsupials  agree  in  possessing  a  pouch, 


Fia.   107. — Duckbills,  and  kangaroo  with  young. 

or  marsupium,  supported  by  a  pair  of  marsupial  bones 
which  extend  forward  from  the  hips.  The  young  are  born 
in  a  very  immature  condition,  transferred  to  the  pouch, 
and  attach  themselves  to  a  teat  by  means  of  a  sucking 
mouth. 

The  opossums  are  confined  to  America,  where  they  are  the 
most  important  of  the  marsupial  animals.  Only  one  species, 
the  Virginia  opossum,  Didelphis  virginiana,  is  commonly 
found  in  the  United  States,  chiefly  in  the  south  and  middle 
west.  This  animal  usually  sleeps  during  the  day  and  comes 


MAMMALIA  291 

out  at  night  to  hunt  for  insects,  berries,  nuts,  small  birds, 
mammals,  eggs,  etc.  Two  or  three  litters  of  four  to  six  are 
produced  annually.  The  young  remain  with  the  mother 
for  a  couple  of  months;  at  first  in  the  pouch,  then  clinging 
to  her  back. 

Order  3.  Edentata.— As  the  name  of  this  order  indi- 
cates, the  animals  included  are  without  teeth.  The 
sloths,  armadillos,  and  the  American  ant  eaters  are  com- 
mon representatives.  The  only  species  entering  the  United 
States  is  the  nine-banded  armadillo  which  occurs  along  the 
Mexican  border.  This  interesting  animal  bears  four  young 
in  each  litter  which  are  always  of  the  same  sex  and  result 
from  the  fragmentation  of  a  single  embryo.  Such  multipli- 
cation of  the  individuals  from  a  single  fertilized  egg  is 
known  as  polyembryony. 

Order  4.  Insectivora. — The  insectivores  are  all  small 
in  size  and  live  for  the  most  part  ^n  or  in  the  ground.  The 
moles  and  shrews  are  common  representatives  in  North 
America.  The  moles  (Family  Talpidce)  are  stout,  with 
powerful  forefeet  suited  for  digging,  rudimentary  eyes, 
and  no  external  ears.  The  common  mole,  Scalops  aquations. 
burrows  just  below  the  surface  of  the  soil  and,  though  it 
sometimes  disfigures  lawns,  does  considerable  good  by 
destroying  insects.  The  shrews  are  tiny,  shy,  mouse- 
like creatures  with  pointed  heads.  Some  species  are  among 
the  smallest  of  mammals.  Their  food  consists  of  insects, 
snails,  worms,  and  other  suitable  objects. 

Order  5.  Chiroptera. — The  bats  belong  here.  The  fly- 
ing squirrels,  and  a  few  other  hairy  animals  are  able  to  sail 
snort  distances,  but  the  bats  are  the  only  mammals  which 
really  fly.  The  digits  on  the  fore  limbs  are  spread  out  to 
support  the  thin  wing  membranes,  the  breast  bone  has  a 
keel  for  the  attachment  of  wing  muscles,  and  there  are 
other  adaptations  for  aerial  locomotion.  Bats  have  re- 
markable ability  to  avoid  obstacles  while  on  the  wing.  An 
individual  in  which  the  eyes  have  been  destroyed  is  able 
to  fly  about  in  a  room  crossed  by  a  number  of  strings  with- 


292  GENERAL  ZOOLOGY 

out  touching  anything.  Young  bats  cling  to  their  mother 
and  are  carried  about  until  able  to  take  care  of  themselves. 

Bats  usually  sleep  during  the  day,  suspended  in  some 
tree,  cave,  or  belfry,  and  come  out  at  night  to  seek  food. 
The  common  bats  in  the  United  States  are  all  insectivorous, 
but  certain  large,  tropical  species  live  on  fruit  and,  in  South 
America,  the  vampires  bite  sleeping  animals  and  feed  on 
their  blood. 

Order  6.  Carnivora. — The  carnivores  may  be  terres- 
trial, arboreal  or  aquatic  in  their  habits,  but  all  have  sharp 
teeth  with  prominent  canines,  and  poorly  developed 
clavicles.  This  order  contains  many  animals  of  great 
interest  to  man — cats,  dogs,  fur-bearers,  etc.  It  is  usually 
subdivided  into  two  suborders:  (1)  Fissipedia,  including 
chiefly  terrestrial  carnivores;  and  (2)  Pinnipedia,  to  which 
the  seals,  walruses,  and  sea-lions  belong.  There  are  a 
number  of  important  families  of  carnivores  in  North 
America.  The  Canidce  walk  on  their  toes  (digitigrade) 
and  have  non-retractile  claws.  The  dogs,  foxes,  wolves, 
and  coyotes  belong  to  this  family.  The  raccoons  (Pro- 
cyonidce)  and  bears  (Ursidce)  walk  on  the  entire  foot 
(i.e.,  are  plantigrade).  The  martens  (Mustelidce)  are 
widespread  in  North  America;  the  forty-six  species  includ- 
ing the  mink,  weasel,  marten,  wolverine,  skunk,  badger, 
and  otter.  Many  aTe  greatly  valued  for  their  fur.  The 
family  Felidce  contains  the  carnivores  with  retractile 
claws — cat,  puma,  leopard,  tiger,  jaguar,  etc.  The  aquatic 
carnivores  (Pinnipedia)  are  greatly  modified  structurally 
for  life  in  the  water.  The  eared  seals  (Otariidce),  walruses 
(Odobcenidce) ,  and  the  earless  seals  (Phocidce)  all  have 
representatives  on  the  shores  of  America. 

Order  7.  Rodentia. — Rodents  possess  strong  incisor 
teeth  fitted  for  gnawing  and  lack  canines.  They  are  usually 
of  small  or  of  moderate  size,  the  jack-rabbit  and  beaver 
being  among  the  largest  known.  In  number  of  species 
this  is  the  largest  order  of  mammals,  over  fourteen  hundred 
being  included.  Important  representatives  in  North 


MAMMALIA  293 

America  are  the  rabbits,  hares  (Leporidce) ;  squirrels, 
prairie-dogs,  woodchucks,  chipmunks,  ground-squirrels, 
flying  squirrels  (Sciuridce) ',  beavers  (Castoridce)-,  pocket- 
gophers  (Geomidce) ;  rats,  mice,  voles,  muskrats  (Muridce) ; 
and  porcupines  (Ccendidce).  The  beavers  are  unique 
among  rodents  in  possessing  a  flat  scaly  tail  which  they 
use  for  swimming.  The  porcupine's  quills  are  peculiar 
structures,  which  are  to  be  looked  upon  as  highly  modified 
hairs.  They  must  be  touched  to  do  injury  and  cannot 
be  shot  out  as  is  sometimes  believed.  Rats  have  recently 
been  the  subject  of  special  interest  because  of  the  discovery 
that  their  fleas  commonly  carry  bubonic  plague. 

Order  8.  Pholidota. — The  scaly  ant  eaters,  or  pan- 
golins, are  peculiar  mammals  inhabiting  Africa  and  parts 
of  Asia.  Their  bodies  are  covered  with  flat  scales  and  they 
roll  themselves  up  into  balls  when  molested,  like  armadillos. 

Order  9.  Primates. — This  order  includes  the  lemurs, 
monkeys,  apes,  and  man.  Its  representatives  are  mostly 
found  in  the  warmer  parts  of  the  earth  and  usually  live 
in  trees.  They  are  well  adapted  to  arboreal  life,  usually 
possessing  thumbs  and  great  toes  opposable  to  the  other 
digits  so  that  the  hands  and  feet  are  admirably  fitted  for 
grasping.  Most  primates  are  somewhat  social  in  their 
habits  and  usually  go  about  in  small  bands.  One  young 
is  usually  born  at  a  time  and  it  is  attended  with  great 
care. 

The  lemurs  (Lemuridce)  are  dog-like  arboreal  animals, 
mostly  confined  to  the  island  of  Madagascar.  They  usually 
have  a  long  non-prehensile  tail  ^and  the  toes  bear  both 
claws  and  flattened  nails.  The  marmosets  (Halpalidce) 
are  found  in  Central  and  South  America.  Their  great  toes 
bear  flat  nails  but  the  others  have  claws;  the  tail  and  ears 
are  long;  the  thumb  is  not  opposable  to  the  other  digits; 
the  brain  is  rather  large;  and  the  space  between  the  nostrils 
wide.  The  South  American  monkeys  (Cebidce)  have  flat 
nails  on  all  the  digits  and  can  oppose  both  the  thumb  and 
great  toe;  the  tail  is  usually  long  and  prehensile;  and  there 


294  GENERAL  ZOOLOGY 

is  a  wide  space  between  the  nostrils.  The  old  world 
monkeys  (Cercopithecidce)  as  a  rule  have  long  tails  which 
are  never  prehensile;  their  buttocks  are  often  covered  with 
thick  callosities  which  are  usually  bright-colored;  the 
nostrils  are  close  together.  Many  of  these  apes  have  cheek 
pouches  for  carrying  food.  They  are  found  in  Africa  and 
Asia. 

The  anthropoid,  or  man-like  apes  (Simiidce),  spend  most 
of  their  time  in  trees,  and  do  not  walk  on  the  palm  of  the 
hand  when  on  the  ground  like  most  monkeys,  but  stand 
more  or  less  erect  on  the  hind  legs  or  walk  in  a  stooping 
position  resting  partly  on  the  backs  of  the  hands.  The 
tail  is  absent  and  the  arms  are  longer  than  the  legs.  There 
are  four  genera  of  anthropoids  which  include  the  gibbons 
(Hylobates),  orang-utan  (Pongo),  gorilla  (Gorilla),  and 
chimpanzee  (Anthropopithecus) .  The  gibbous  are  about 
three  feet  high  and,  though  possessing  very  long  arms,  do 
in?  t  assist  themselves  with  the  hands  when  walking.  They 
are  found  in  Eastern  Asia.  The  orang-utans  live  in  Borneo 
and  Sumatra.  They  use  the  knuckles  of  the  hands  in 
walking,  and  build  platforms  of  sticks  in  trees.  The  gorilla 
is  a  native  of  West  Africa,  where  it  lives  in  trees  and  feeds 
chiefly  on  vegetation.  It  may  reach  a  height  of  five  and 
a  half  feet  and  weigh  five  hundred  pounds.  The  chim- 
panzee lives  in  the  same  region  but  is  smaller  than  the 
gorilla.  It  is  more  like  man  than  any  other  living  mammal. 
It  is  easily  tamed,  and  has  often  been  trained  to  perform 
various  simple  tasks. 

The  family  Hominidce  includes  only  one  species,  Homo 
sapiens,  or  man.  This  is  distinguished  from  other  man- 
like apes  by  the  erect  walk,  power  of  articulate  speech,  and 
marked  ability  to  reason.  There  are  three  great  races  of 
men:  (1)  the  Negroid,  with  dark  skin,  curly  hair,  flat  nose, 
thick  lips,  prominent  eyes,  and  large  teeth;  (2)  the  Mon- 
golian, with  black  straight  hair,  yellowish  skin,  broad  face 
with  prominent  cheek  bones,  a  small  nose,  sunken,  narrow 
eyes,  and  teeth  of  moderate  size;  and  (3)  the  Caucasian, 


MAMMALIA  295 

having  soft  straight  hair,  vigorous  beard,  retreating  cheek 
bones,  narrow  prominent  nose,  and  small  teeth. 

Order  10.  Artiodactyla. — This  large  order  includes  the 
hoofed  animals  with  an  even  number  of  toes.  There  are 
many  that  are  of  economic  importance,  seFving-^as-  fur  or 
food  for  man  and  as  his  domestic  animals.  The  pigs, 
peccaries,  and  hippopotami  do  not  chew  a  cud,  but  rumi- 
nants (camel,  deer,  giraffe,  prong-horn  antelopes,  and  cattle) 
crop  herbage  hastily  without  masticating  it  enough  for 
digestion  and  regurgitate  it  into  the  mouth  later  for 
thorough  chewing.  A  typical  ruminant  has  the  stomach 
divided  into  four  parts,  the  functions  of  which  are  corre- 
lated with  the  peculiar  habits  of  feeding.  The  deer  have 
solid  horns  which  are  shed  annually  ^but  the  cattle  have 
horns  with  a  vascular  bony  core  and  a  hard  outer  sheath.. 
The  prong-horn  antelope  of  Western  North  America  is 
unique,  having  horns  like  cattle  but  shedding  them  annu- 
ally. The  descendants  of  the  British  Bos  taurus  are  the 
commonest  of  the  domestic  cattle,  though  the  water 
buffalo,  yak,  and  others  are  important  in  certain  districts. 

Order  11.  Perissodactyla. — The  odd-toed  hoofed  ani- 
mals include  the  horses  (Equidce),  tapirs,  and  rhinoceroses. 
Though  none  of  the  Equidse  were  found  in  America  when 
Columbus  discovered  it,  most  of  the  past  evolution  of  the 
family  took  place  in  the  United  States.  The  horses  show 
extreme  reduction  of  the  pentadactyl  limb ;  only  the  middle 
toe  is  functional  and  the  terminal  hoof  represents  the  toe- 
nail.  At  the  present  time  there  are  over  sixty  varieties  of 
domesticated  horses,  all  belonging  to  one  species,  Equus  ca- 
ballus.  The  asses,  zebras,  and  quaggas  still  run  wild  in 
Africa  and  Asia,  but  several  species  have  been  tamed. 

Order  12.  Proboscidia. — The  two  living  spebies  of  ele- 
phants inhabit  Africa  and  India*  respectively.  Both  have 
five  toes  on  all  the  feet,  the  characteristic  elongated  trun^: 
with  the  nostrils  opening  at  its  tip,  and  a  thick  loose  skin 
(whence  the  name,  " pachyderm").  Elephants  have  no 
canine  teeth;  the  tusks  are  elongated  incisors.  In  rather 


296  GENERAL  ZOOLOGY 

recent  geological  times  certain  fossil  elephants,  like  the 
mastodon  and  mammoth,  lived  in  the  Northern  Hemi- 
sphere. 

Order  13.  Sirenia. — The  manitees  and  dugongs  are 
large  aquatic  animals  which  live  in  shallow  waters  along 
the  sea  shore  and  in  the  mouths  of  rivers.  These  great 
animals  live  on  vegetation  and  the  bones  are  very  heavy  so 
that  they  may  remain  on  the  bottom  without  effort.  Their 
limbs  are  modified  to  form  flippers  which,  with  the  tail, 
serve  for  swimming. 

Order  14.  Cetacea. — There  are  two  kinds  of  whales: 
(1)  those  with  teeth  (Odontoceti) ;  and  (2)  those  in  which 
the  teeth  are  never  functionally  developed,  being  present 
in  the  young  but  replaced  by  baleen  or  "  whalebone"  in  the 
adult  (Mystacoceti).  Though  whales  are  fish-like  in 
general  form  and  wholly  aquatic  in  their  habits,  they  are 
true  mammals.  The  body  before  birth  has  a  thick  coating 
of  hair,  and  the  young  are  nourished  with  secretions  from 
the  mammary  glands.  A  whale's  nostrils  open  through 
the  "  bio  whole"  on  the  top  of  the  head,  and  a  great  cloud 
of  spray  and  vapor  is  spouted  from  this  aperture  when  it. 
comes  to  the  surface  to  breathe.  The  toothed  whales  in- 
clude the  dolphins,  porpoises,  grampuses,  killer  whales, 
sperm  whales,  and  narwals.  The  baleen  whales  are  as  a 
rule  of  large  size,  including  the  great  rorquals,  fin-whales, 
hump-backed,  and  right  whales.  " Whalebone"  hangs 
from  the  roof  of  a  whale's  mouth  in  long  plates  which  are 
frayed  out  at  the  free  ends.  In  feeding,  water  is  taken  into 
the  mouth  and  squirted  out  between  the  plates,  thus 
straining  out  many  small  animals.  The  sulphur-bottom 
whale,  Balcenoptera  sulfureus,  is  the  largest  living  animal. 
It  may  attain  a  length  of  ninety-five  feet  and  weigh  nearly 
300,000  pounds. 

Probably  the  greatest  interest  which  any  man  has  in 
mammals  relates  to  his  own  relationships  to  animals  in 
general.  The  next  two  chapters  will  accordingly  be  devoted 
to  man  as  an  animal  and  his  place  in  the  animal  kingdom. 


CHAPTER  XXVII 

MAN,  Homo  Sapiens  Linnaeus 

*  The  proper  study  of  mankind  is  man." — Pope. 

Though  zoologists  were  at  first  loath  to  admit  their  re- 
lationship to  other  monkey-like  animals  and  for  a  time 
placed  man  in  a  distinct  order  of  mammals,  they  have  been 
forced  by  truth-seeking  science  to  connect  him  closely  with 
other  primates.  All  types  of  men  living  at  the  present 
time  are  believed  to  belong  to  a  single  species,  Homo  sapiens, 
which  is  the  only  one  in  the  family  Hominidce.  The 
various  races  which  are  united  in  this  species  differ  some- 
what from  each  other  but  also  have  much  in  common.  An 
individual  from  any  one  race  is  fertile  in  breeding  with  a 
representative  from  another,  and  there  are  no  fundamental 
structural  differences.  Man  must  be  looked  upon  as  an 
animal  belonging  to  the  primates  and  showing  closer  simi- 
larities to  his  nearest  relatives  (chimpanzee,  gorilla,  gibbon, 
orang-utan)  than  they  do  to  other  monkeys. 

Like  all  other  animals,  man's  body  is  a  machine  which  is 
controlled  by  such  structures  as  levers,  pulleys,  muscles, 
sense  organs,  and  nerves — all  of  which  conform  in  their 
operation  to  the  laws  of  chemistry  and  physics.  His  body 
is  made  up  of  cells  which  are  like  those  of  other  animals;  it 
originates  by  growth  and  cell-division  from  a  single  cell— 
the  fertilized  egg;  and  its  cells  show  differentiation  into 
tissues,  corresponding  to  a  similar  division  of  labor  in  other 
vertebrates.  There  are  many  who  believe  that  man  differs 
from  all  other  animals  in  possessing  a  soul,  or  spirit,  which 
makes  him  unique.  At  the  present  time,  science  cannot 
state  definitely  that  man,  or  any  other  animal,  possesses  or 
lacks  a  "soul,"  though  there  is  often  heated  argument 
with  eminent  scientific  men  on  both  sides.  But  this 

297 


298  GENERAL  ZOOLOGY 

question  does  not  properly  come  within  the  scope  of  this 
book  and  our  attention  will  be  directed  to  the  more  strictly 
zoological  aspects  of  man's  activities. 

Self-maintenance. — Judging  by  the  structure  of  the 
teeth  and  other  parts  concerned  with  nutrition,  man  is 
fitted  to  live  on  a  diet  consisting  of  fruits,  herbs,  and  flesh. 
Savages  often  eat  much  of  their  food  raw  and  devour 
things  which  have  little  appeal  to  civilized  tastes.  The 
American  Indian  revels  in  dog  feasts;  the  primitive 
Australians  esteem  the  luscious  caterpillar  as  a  great 
delicacy.  Each  race  has  certain  food  customs  which  are 
adhered  to  more  or  less  strictly.  The  Italian  loves  his 
spaghetti;  the  Irishman  relishes  potatoes;  the  Australian 
prays  to  his  gods  that  caterpillars  may  be  abundant;  the 
Hawaiian  subsists  largely  on  crabs  and  fish. 

Though  a  man's  body  may  be  maintained  for  a  con- 
siderable time  by  eating  nothing  but  protein  food,  it  thrives 
best  on  a  varied  diet  including  proteins,  carbohydrates  and 
fats.*  Men  forced  to  live  without  vegetables  often  have 
the  scurvy  or  other  similar  diseases  due  to  improper  nutri- 
tion. Recently  it  has  been  discovered  that  certain  foods 
contain  very  small  quantities  of  substances  (so-called 
vitamines,  etc.)  which  play  a  very  important  role  in  nutri- 
tion, though  they  furnish  very  little  actual  building  material 
or  energy.  For  example,  people  living  largely  on  polished 
rice  may  be  attacked  by  the  disease  known  as  beri-beri ; 
whereas  those  eating  the  same  proportion  of  rice  which  has 
the  outer  brown  covering  are  without  such  trouble.  In 
the  fat  of  butter  there  are  minute  quantities  of  another 
substance  which  stimulates  growth.  To  have  the  greatest 
value,  proteins,  carbohydrates,  and  fats  should  occur  in 
fairly  definite  proportions.  The  daily  requirements  for 
an  average  man  as  estimated  by  Atwater  are  as  follows: 

Protein 125  grammes  (4.41  oz.) 

Fats 125  grammes  (4.41  oz.) 

Carbohydrates 400  grammes  (14.11  oz.) 

*  For  definitions  of  these  substances,  see  page  30. 


MAN  299 

The  protein  is  used  primarily  for  building  body  substance, 
and  nitrogen  is  its  most  important  chemical  element. 
Carbohydrates  and  fats  furnish  energy  to  carry  on  the  work 
necessary  for  the  activity  of  the  body,  and  their  important 
element  is  carbon.  If  the  daily  ration  is  poor  in  protein, 
carbohydrates  and  fats  cannot  make  up  the  deficiency; 
if  it  is  lacking  in  carbonaceous  substances,  the  proteins 
must  be  broken  down  in  excess  to  release  energy.  In 
addition  to  organic  foods,  water  and  particular  mineral 
salts  are  of  course  necessary. 


Lean  Beef 
L  ean  smoked  ham 
Bacon  smoked 
Fresh  codfish 


Milk 

Buffer 

Oatmeal 

Rice 

IVhite  bread 

Dried  beans 

String  beans 

Cabbage 

Green  corn 

Potatoes 

L  e/Tuce 

Tomatoes 

Apples 

Almonds 

Chestnuts 

Peanuts 


\SMHProtein    I        Varhohydrate 

FIG.  108. — Constituents  of  common  foods.     (From  data  in  Sherman's  Chemistry 
of  Food  and  Nutrition.) 

Man  is  well-equipped  to  seek  and  capture  food.  A 
savage  depends  mostly  upon  his  sight  and  hearing  to  dis- 
cover suitable  plants  or  animals,  though  the  senses  of 
touch,  taste,  and  smell  are  often  employed.  The  organs 
of  smell  are  somewhat  degenerate  and  far  less  effective 
than  in  many  other  mammals,  such  as  the  dog  or  deer. 
The  pos'tion  of  the  eyes  compensates  for  this  deficiency, 
however,  by  giving  man  the  advantage  of  binocular  vision 
which  permits  accurate  judgment  of  distances  to  objects. 
The  legs  are  well-suited  for  pursuing  prey,  and  in  some 
primitive  races  (Igorrotes)  are  used  also  for  holding  objects. 


300  GENERAL  ZOOLOGY 

The  hands  are  probably  the  best  organs  for  seizing  and 
holding  that  any  animal  possesses.  They  are  equally 
effective  for  strangling  a  rabbit,  holding  a  spear,  or  grasp- 
ing a  hoe,  and  the  flat  nails  make  it  possible  to  pick  up 
minute  objects.  The  instinct  for  survival  is  strong  in 
man  and  impels  him  to  keep  his  body  in  training  so  that 
he  may  not  fail  in  the  chase.  His  superior  mentality  gives 
him  a  great  advantage  over  other  animals  in  .securing  food 
and  he  has  been  able  to  invent  many  means  to  insure  a 
continuous  supply. 

Food,  once  in  the  hand,  is  placed  in  the  mouth  and  goes 
the  usual  course — being  chewed,  swallowed,  mixed,  digested 
and  absorbed.  In  the  mouth  food  is  mixed  with  saliva 
which  contains  the  ferment  ptyalin,  capable  of  changing 
starch  to  sugar.  As  soon  as  the  chewed  food  enters  the 
stomach,  gastric  juice  begins  to  pour  out,  but  since  the 
food  remains  in  the  upper  part  of  the  stomach  for  a  time, 
ptyalin  continues  to  act  for  about  half  an  hour.  Later  it 
is  neutralized  by  the  free  hydrochloric  acid  in  the  stomach. 
In  addition  to  its  digestive  functions  the  acid  acts  as  an 
antiseptic,  killing  bacteria  and  rendering  certain  other 
injurious  substances  innocuous.  Gastric  juice  contains  a 
protein-digesting  ferment  (pepsin)  which  liquefies  the  food 
by  dissolving  the  nitrogenous  substances.  Most  of  the 
mixing  and  churning  is  done  near  the  muscular  outlet  of 
the  stomach  and  the  liquefied  food  is  gradually  permitted 
to  pass  the  sphincter*  muscle  which  guards  this  opening. 
In  the  intestine  muscular  mixing  movements  continue  and 
secretions  from  two  great  digestive  glands  (liver,  pancreas) 
are  mixed  with  the  liquefied  food.  Pancreatic  juice  con- 
tains three  powerful  digestive  ferments:  (1)  trypsin,  for 
digesting  proteins;  (2)  amylopsin,  converting  starches  to 
sugars;  and  (3)  steapsin,  which  breaks  up  and  emulsifies 
fats.  In  the  mouth  the  saliva  has  a  slightly  alkaline  reac- 
tion. In  the  stomach  the  food  becomes  acid;  is  slightly 
alkaline  or  neutral  after  entering  the  intestine,  but  later 

*  A  sphincter  is  a  ring-like  mass  of  muscle  which  surrounds  an  opening. 


MAN  301 

becomes  acid  again.  The  liquefiable  nutriment  is  gradually 
extracted  from  the  food  and  the  residue  of  undigested 
material  is  finally  eliminated  through  the  anus.  In  a 
healthy  man  digestion  is  usually  influenced  by  numerous 
bacteria  which  live  in  the  alimentary  canal.  Some  in- 
vestigators assert  that  these  unsuspected  guests  are 
absolutely  indispensable  for  proper  digestion. 

The  absorption  of  food  is  selective — not  simply  a  soaking 
of  liquids  through  the  lining  of  the  alimentary  canal.  .If 
fresh  blood  is  placed  in  the  intestine  it  is  not  transferred 
unchanged  to  the  adjacent  blood-vessels  but  goes  the  usual 
long  round  of  digestion,  absorption,  and  assimilation.  The 
proteins  are  mostly  changed  to  amino  acids  and  usually 
pass  through  the  lining  of  the  intestine  into  the  blood-ves- 
sels; the  fats  are  emulsified,  broken  up  into  fatty  acids 
and  glycerin  and  taken  up  by  the  lac  teals;*  the  carbo- 
hydrates for  the  most  part  pass  as  monosaccharids  in 
the  blood  to  the  liver,  where  they  are  stored  as  glycogen 
or  transferred  to  the  tissues  to  be  burned  for  energy 
production. 

Most  of  the  activities  concerned  with  digestion  are 
under  the  control  of  the  nervous  system,  but  usually  take 
place  without  the  knowledge  of  the  person  who  has  eaten. 
Vomiting  is  a  safeguard  for  rejecting  poisonous  or  unde- 
sirable substances;  headache  may  be  an  indication  of  an 
irritated  stomach.  Yet  many  of  the  digestive  and  assimi- 
lative processes  are  controlled  by  specific  chemical  sub- 
stances, which  may  not  even  enter  the  alimentary  canal. 
The  pancreas  not  only  manufactures  a  powerful  digestive 
secretion  which  it  pours  into  the  intestine,  but  also  acts 
as  a  ductless  gland  which  is  of  great  importance  in  con- 
nection with  the  utilization  of  sugar  from  the  blood.  When 
it  is  diseased  the  metabolism  of  carbohydrates  is  accord- 
ingly disturbed.  Other  ductless  glands  (thyroid,  thymus, 
adrenals,  gonads,  etc.)  all  have  more  or  less  influence  on 

*  Lacteals  are  branches  of  the  lymphatic  system  which  form  a  network 
about  the  digestive  organs. 


302  GENERAL  ZOOLOGY 

metabolic  processes,  though  they  take  no  direct  part  in 
the  digestion  of  food. 

The  circulatory  system  of  man  is  a  highway  for  the 
transfer  of  food,  waste  products,  gases,  and  the  leucocytes 
which  destroy  undesirable  materials.  The  food  in  the 
liquid  portion  of  the  blood  is  largely  protein.  Carbohy- 
drates and  fats  are  stored  in  convenient  places  within  the 
body  and  enter  the  blood  in  small  quantities  as  needed. 
The  red  corpuscles  carry  oxygen  from  the  lungs  to  the 
tissues,  and  the  blood  returns  to  the  lungs  laden  with  carbon 
dioxide.  The  heart  pulsates  rhythmically  for  the  entire 
life  of  a  man,  without  rest,  except  what  it  snatches  between 
beats.  It  forces  blood  through  the  arteries  to  all  parts 
of  the  body,  and  is  continually  filled  from  the  veins. 
Though  the  rhythm  of  pulsation  is  under  the  control  of  the 
nervous  system,  the  tendency  to  beat  is  inherent  in  certain 
portions  of  heart  muscle  and  movements  may  be  induced 
or  modified  by  the  presence  of  certain  salts  in  the  blood  or 
by  other  stimuli.  Through  the  delicate  walls  of  the  capil- 
laries, which  connect  the  arteries  and  veins,  food  and 
oxygen  are  supplied  to  the  tissues  and  waste  material 
enter  the  blood.  Though  the  amount  of  blood  passing 
through  the  heart  may  be  the  same  at  different  times,  the 
vasomotor  nerves  vary  the  diameter  of  the  small  vessels 
so  that  different  parts  of  the  body  have  more  or  less  blood 
as  their  necessities  demand. 

The  respiratory  system  is  concerned  primarily  with  the 
supplying  of  oxygen  to  the  tissues  and  the  elimination  of 
carbon  dioxide.  Air  is  drawn  into  the  lungs  w7hen  the  space 
in  the  thoracic  cavity  is  increased  by  the  contraction  and 
descent  of  the  diaphragm  together  with  the  raising  of  the 
ribs  by  the  shortening  of  the  muscles  between  them. 
Four  hundred  and  forty  liters  (888  gallons)  of  blood  pass 
through  the  lungs  each  day.  To  aerate  this  an  adult  man 
requires  about  85,000  liters  (3000  cubic  feet)  of  air  per  hour. 
The  atmosphere  may  contain  1  to  2  per  cent,  of  carbon 


MAN  303 

dioxide  and  still  be  fit  for  breathing,  but  a  greater  amount 
is  injurious. 

The  excretions  formed  as  a  result  of  metabolism  are 
chiefly  eliminated  through  the  kidneys,  lungs,  and  skin. 
The  kidneys  discharge  the  urea,  formed  as  one  of  the  end 
products  when  proteins  break  down.  Though  urea  is 
excreted  through  the  kidneys,  it  is  formed  for  the  most 
part  in  the  liver  and  transferred  in  the  blood.  Water  passes 
out  through  all  three  of  the  excretory  channels;  carbon 
dioxide  is  eliminated  largely  through  the  lungs,  but  some 
goes  through  the  skin.  All  the  excretory  organs  are  con- 
trolled and  coordinated  by  the  nervous  system.  For 
example,  in  cool  weather  there  is  less  water  passing  out 
through  the  skin  and  more  through  the  kidneys. 

The  activities  of  the  human  body  are  more  or  less  rhyth- 
mical. The  day  is  usually  a  time  of  activity  and  the  body 
is  used  up  to  some  extent  in  doing  work;  night  is  a  period 
of  rest  and  losses  are  made  good  during  sleep.  Sleep  is  a 
curious  phenomenon — sense  organs  which  are  at  other 
times  quick  to  receive  stimuli  become  inactive  and  all 
extracorporal  activities  cease.  There  have  been  a  number 
of  theories  to  explain  sleep  and  the  three  following  may  be 
mentioned:  (1)  Sleep  may  be  caused  by  the  using  up  of  sub- 
stances necessary  for  nervous  activity;  (2)  it  may  be  due 
to  the  accumulation  of  waste  products  which  cause  the 
enlargement  of  small  blood-vessels  throughout  the  body 
with  a  resulting  scarcity  of  blood  in  the  brain  (though  such 
changes  do  occur,  they  probably  do  not  cause  sleep);  (3) 
sleep  may  be  due  to  the  contraction  of  nerve  branches  so 
that  those  of  different  cells  are  no  longer  in  contact — thus 
the  paths  are  broken  along  which  nervous  impulses  travel 
when  the  body  is  awake.  Whatever  its  cause,  sleep  is 
necessary  for  all  warm-blooded  animals,  and  the  human 
machine  is  most  efficient  when  periods  of  rest  and  activity 
alternate  with  reasonable  regularity. 

Self-protection. — The  instinct  for  self-protection  is  as 
strong  in  man  as  in  other  animals  and  the  body  has  many 


304  GENERAL  ZOOLOGY 

natural  adaptations  for  protection.  Even  its  cells  and 
fluids  have  properties  which  effectively  shield  them  from 
dangers  likely  to  be  encountered.  The  skin  usually  keeps 
out  undesirable  materials.  If  it  is  broken,  however,  the 
clotting  of  the  blood  forms  an  effective  plug  which  checks 
contamination  from  the  outside.  If  bacteria  or  poisons 
do  gain  an  entrance,  they  are  eaten  up  by  the  leucocytes 
(which  swarm  through  the  blood  channels  to  repel  invaders), 
destroyed  by  antitoxins,  or  oxidized.  Exposed  parts  of 
the  body  are  especially  protected  by  heavy  growths  of  hair 
or  callouses.  Civilized  man  has  devised  clothing,  houses, 
and  other  protective  devices  to  assist  his  natural  defences. 
Man  is  also  one  of  the  favored  animals  which  have  a  con- 
stant body  temperature,  maintained  by  delicate  adjust- 
ments and  coordinations  of  metabolic  activities,  vaso- 
motor  nerves,  and  perspiratory  glands.  He  is  thus  able 
to  survive  critical  periods  of  climatic  stress,  when  a  cold- 
blooded animal  would  die.  Man  is  also  endowed  with 
discriminating  courage,  reasonable  fear,  and  a  great  degree 
of  resourcefulness,  so  that  he  may  fight,  flee,  or  escape 
danger  by  using  his  wits. 

Yet,  despite  all  man's  versatility  in  combating  dangers, 
he  is  continually  subject  to  minor  disorders  and  is  often 
killed  outright  by  accident  or  disease.  There  are  many 
chances  for  the  occurrence  of  defects  in  the  bodily  machine. 
(1)  A  man  may  be  born  with  some  defect  which  cannot  be 
corrected — hunchback  is  an  example  of  such  an  affliction. 
Among  the  most  pathetic  of  such  cases  are  those  in  which 
the  nervous  system  is  defective.  There  is  a  malady  known 
as  Little's  disease  in  which  the  body  and  mind  may  be 
perfect,  but  the  nervous  connections  (pyramidal  tracts) 
never  grow  down  completely  from  the  brain,  as  they  should 
do  about  the  time  of  birth.  The  bodily  movements, 
therefore,  are  not  coordinated  because  messages  cannot  be 
properly  carried  from  the  brain.  A  bright  mind  may  thus 
wear  itself  out  in  a  helpless  but  perfect  body  over  which 
it  has  little  or  no  control,  and  which  ultimately  dies  from 


MAN  305 

lack  of  exercise.  This  disease  is  due  to  nothing  but  a 
lack  of  development  and  cannot  be  treated  in  any  way. 
(2)  A  defect  may  be  acquired  through  accident.  Man  has 
small  power  of  regeneration  when  compared  with  many 
other  vertebrates.  A  salamander  can  replace  a  lost  leg 
but  a  man  cannoc  grow  a  single  joint  of  a  finger.  Specializa- 
tion has  gone  so  far  that  the  tissues  remaining  can  at  best 
only  close  the  wound.  Accidents  which  maim  or  impair 
the  nervous  system  are  most  serious;  a  sudden  shock  or 
strain  may  result  in  paralysis,  insanity,  or  other  perma- 
nent disorders.  (3)  The  machine  may  be  without  seri- 
ous structural  defects  but  fail  to  operate  properly.  The 
digestive  fluids  may  be  too  acid;  excretions  may  not  be 
properly  eliminated  and  poison  the  tissues;  the  mind  may 
not  form  sound  deductions  from  observed  phenomena. 
(4)  Many  occupations  are  conducive  to  specific  diseases — 
the  Indian  leads  an  active  life  with  much  exposure  and  has 
his  old  age  made  unpleasant  by  rheumatism;  the  coal  miner 
has  his  lungs  impaired  by  dust  and  bad  air,  hence  often 
acquires  tuberculosis;  the  sedentary  business  man  is  dogged 
by  insomnia  and  indigestion.  (5)  Clothing  may  be  a 
source  of  diseases — improper  shoes  often  cause  permanently 
deformed  feet;  corsets  may  cause  serious  disturbances  in 
the  digestive,  circulatory,  or  reproductive  organs. 

All  the  disorders  mentioned  up  to  this  time  are  due  to 
some  defect  in  man's  structural  mechanism  or  in  its  opera- 
tion, but  there  is  another  class  of  diseases  (6)  which  are 
due  to  plant  or  animal  parasites  which  actively  invade  the 
body.  The  chief  avenues  for  the  entrance  for  such  un- 
desirable organisms  are  through  the  natural  openings  in 
the  skin,  though  some  parasites  enter  in  other  ways  (like 
the  tetanus  bacillus  which  gains  access  through  cuts,  and 
malaria  which  is  injected  hypodermically  by  mosquitoes). 
Such  diseases  as  cholera,  typhoid,  trichinosis,  and  tape- 
worms, enter  through  the  mouth  with  the  food ;  diphtheria, 
infantile  paralysis,  tuberculosis,  and  some  other  diseases 
are  usually  spread  through  nasal  discharges;  the  eggs  of 


306  GENERAL  ZOOLOGY 

certain  parasitic  worms  pass  out  in  the  urine  and  hatch 
out  larvae  which  bore  through  the  skin;  some  diseases 
(syphilis,  gonorrhoea)  are  transmitted  through  the  contact 
of  mucous  membranes  with  those  of  infected  persons  and 
are  usually  acquired  by  kissing  or  through  sexual  relations. 
Small  wonder,  then,  that  with  so  many  opportunities  to 
acquire  diseases,  measures  are  generally  taken  to  combat 
them. 

Modern  medicine  attacks  disease  along  three  general 
lines.  (1)  Preventive  medicine  educates  the  public.  It 
spreads  knowledge  as  to  the  nature  of  prevalent  diseases 
and  tries  to  lessen  chances  for  their  dissemination  by  im- 
proving sanitation,  conditions  of  work,  and  diet.  (2) 
Corrective  measures  attempt  to  help  the  body  to  work 
properly.  Operative  surgery  adjusts  structural  defects; 
physic  retains  water  in  the  undigested  food  and  stimulates 
the  flow  of  water  into  the  intestine;  strychnine  in  small 
quantities  sometimes  serves  as  a  tonic  to  a  fagged-out 
nervous  system  and  allows  it  to  recover  its  normal  efficiency; 
a  change  in  diet  may  alleviate  some  specific  irritation ;  lenses 
may  correct  defective  vision.  (3)  The  killing  or  preven- 
tion of  internal  parasites  is  accomplished  in  various  ways. 
Quinine  is  effective  in  poisoning  malarial  parasites  in  the 
blood  corpuscles;  thymol  given  judiciously  will  kill  hook- 
worms; the  salts  of  mercury  are  injurious  to  the  organisms 
causing  syphilis.  The  human  body  has  within  itself  certain 
reactions  which  are  taken  advantage  of  in  preventing  dis- 
ease. Immunity  to  diseases  may  be  natural  or  acquired. 
Most  persons  will  not  grow  the  virus  of  infantile  paralysis 
in  their  bodies,  thus  showing  a  natural  immunity.  A  person 
who  has  been  sick  with  smallpox  will  not  contract  the  dis- 
ease again;  he  has  acquired  immunity  because  his  body  has 
formed  antitoxins  which  prevent  the  growth  of  smallpox 
organisms.  Modern  medicine  has  made  it  possible  to 
acquire  immunity  to  certain  diseases  by  contracting  them 
in  a  mild  form  through  vaccination.  Under  such  circum- 
stances the  body  during  light  attacks  of  disease  develops 


MAN  307 

antitoxins  which  prevent  the  entrance  of  more  serious 
infections. 

Frequently  the  most  difficult  task  of  the  physician  is  to 
tell  what  is  the  matter  with  his  patient.  The  human  body 
is  such  a  complicated  mechanism  that  symptoms  may 
come  by  very  circuitous  routes  and  be  difficult  of  diagnosis. 
Doctors  are  often  blamed  for  not  giving  proper  treatment 
when  they  have  been  obliged  to  choose  from  two  or  three 
possibilities.  It  is  unfortunate  that  ability  to  diagnose 
correctly  is  usually  difficult  to  acquire  and  comes  only  with 
wide  knowledge  and  long  experience,  while  the  mere 
treatment  of  most  diseases  is  comparatively  easy. 

Race  Preservation. — Herter*  says:  "the  sex  instinct  is 
the  only  human  instinct  that  can  be  compared  with  the 
instinct  of  self-preservation  in  respect  to  the  profundity 
of  its  influence  on  the  conduct  of  man."  The  activities 
associated  with  mating,  loving,  and  the  rearing  of  offspring 
dominate  the  life  of  every  normal  adult  individual.  Though 
sex-determination  perhaps  depends  only  upon  the  presence 
or  absence  of  an  accessory  chromosome  in  the  sperm  cell 
which  fertilizes  the  egg  and  thus  initiates  a  new  human 
being,  the  two  sexes  show  fundamental  differences  in  their 
instincts. 

A  man  is  larger  and  stronger  than  a  woman.  His 
secondary  sex  characters  (beard,  deep  voice,  great  strength, 
and  more  aggressive  disposition)  fit  him  to  hunt,  fight,  or 
endure  other  privations  necessary  for  the  maintenance 
of  a  family.  Woman,  with  her  soft  voice,  mammary  glands, 
and  disposition  to  foster  and  cherish,  is  anchored  to  the 
central  interest  of  family,  forcing  her  to  a  relatively  sessile 
life.  The  structural  differences  between  the  sexes  are 
associated  with  characteristic  tastes  and  habits.  "The 
essential  differences  between  the  mental  life  of  woman 
and  that  of  man  apparently  depends  upon  the  fact  that  the 
cerebral  organization  represents  and  reflects  the  various 
aspects  of  the  sexual  functions,  which  irradiate  as  it  were, 

*  "Biological  Aspects  of  Human  Problems,"  N.  Y.,  1911. 


308  GENERAL  ZOOLOGY 

into  the  brain.  The  circumstances  that  the  body  of  the 
mother  nourishes  both  the  embryo  and  the  infant  brings 
her  into  an  organic  relationship  to  family  different  to.  the 
casual  relationship  of  the  male.'7* 

In  the  choosing  of  a  mate  the  initiative  is  taken  by  the 
man.  The  primitive  native  of  New  Guinea  adopts  the 
simple  expedient  of  killing  the  man  who  already  possesses 
the  woman  he  wants.  In  China  and  Turkey  women  are 
sold  to  men  who  may  desire  them.  If  men  are  to  do  the 
choosing,  women  must  be  attractive  in  order  to  be  selected, 
and  the  gentler  sex  has  developed  charming  qualities,  both 
natural  and  artificial.  The  New  Guinea  woman  dresses 
her  hair' by  rolling  strands  in  bright-colored  clays  in  order 


/ 

FIG.   109. — The  races  of  men — Mongolian,  Caucasian,  Negroid. 

that  she  may  be  more  beautiful.  Civilized  women  show 
the  same  instinct  in  their  elaborate  dresses,  bright  hats, 
ear  rings,  necklaces,  and  other  ornaments.  Forel,  the 
eminent  Swiss  scientist,  believed  that  many  of  the  basic 
psychological  processes  of  the  human  mind  had  their  origin 
in  the  desire  of  the  two  sexes  to  appear  well  in  each  other's 
eyes. 

A  human  being  follows  the  same  course  of  development 
as  other  metazoans.  In  a  mature  woman  an  egg  cell  leaves 
the  ovary  and  starts  down  the  oviduct  about  every  four 
weeks.  If  fertilization  does  not  occur  within  about  two 
weeks,  the  egg  cell  passes  out  and  is  later  succeeded  by 
another.  If  a  sperm  cell  unites  with  an  egg  cell,  the  result- 
ing zygote  adheres  to  the  mucous  lining  of  the  uterus  and 
development  begins.  After  the  usual  cleavage  stages 
(Fig.  64,  page  152)  a  little  streak  (the  primitive  streak) 

*  "Biological  Aspects  of  Human  Problems,"  N.  Y.,  1911. 


MAN  309 

appears  on  one  side  of  the  germ,  and  later  a  groove,  which 
is  the  beginning  of  the  nervous  system,  forms  down  its 
center.  The  germ  layers  (ectoderm,  entoderm,  mesoderm) 
are  soon  formed,  the  enteron  and  notochord  take  form, 
and  at  the  end  of  the  third  week  the  heart  of  the  embryo 
begins  to  pulsate  (Fig.  115). 

The  human  embryo  has  unusual  opportunities  for  de- 
velopment. It  is  nourished  for  nine  months  in  the  uterus 
of  its  mother  from  her  blood  and  rests  in  watery  fluids 
surrounded  by  protective  membranes  (Fig.  106,  page  288). 
The  fully  formed  embryo,  or  fcetus,  does  not  get  blood 
directly  from  its  mother.  There  is  a  transfer  of  nourish- 
ment and  oxygen  through  thin  membranes  in  the  placenta. 
The  blood  in  the  foetal  circulation  is  very  poor  in  oxygen 
and  the  fcetus  is  remarkably  resistant  to  asphyxiation. 
This  is  a  valuable  adaptation  for  it  prevents  suffocation  at 
the  time  of  birth  when  the  circulation  from  placenta  to 
child  is  often  cut  off  completely  for  some  time.  While 
within  the  mother  the  blood  of  the  foetus  passes  through 
the  foramen  ovale,  an  opening  which  makes  a  short  cut 
between  the  two  sides  of  the  heart.  There  are  other  short 
routes  so  that  most  of  the  oxygen  in  the  blood  goes  to  the 
liver,  heart,  and  head.  At  birth  these  channels  close  and 
there  are  fundamental  changes  in  circulation  and  respira- 
tion. Sometimes  some  of  them  fail  to  close  properly  and 
a  "blue  baby"  or  other  defective,  which  usually  dies  soon, 
results. 

During  the  growth  of  the  child  the  mammary  glands  of 
the  mother  have  been  changing  and  soon  after  birth  they 
begin  to  secrete  milk.  This  is  a  true  glandular  secretion 
and  not  merely  "filtered  blood."  A  child,  then,  like  other 
mammals,  has  unusually  advantageous  conditions  for 
development  in  that  it  may  attain  an  age  of  nearly  two 
years  (after  fertilization)  without  having  any  nourishment 
except  that  supplied  by  the  mother.  At  about  the  time 
of  birth  important  nervous  connections  are  established  by 
the  growth  of  certain  tracts  of  nerves  through  the  brain  and 


310  GENERAL  ZOOLOGY 

spinal  cord.  A  child  at  birth  is  rather  helpless  and  does 
not  correlate  its  movements  well,  but  is  nourished,  cared 
for,  and  protected  during  this  critical  period.  It  also 
receives  education,  which  even  in  the  most  savage  races  far 
exceeds  that  obtained  by  any  other  animal.  It  is  taught 
to  walk,  talk,  get  food,  care  for  itself,  and  to  make  imple- 
ments. No  wonder  that  man  has  been  able  to  dominate 
the  earth. 

The  most  important  structural  changes  which  occur  in  a 
growing  child  after  it  ceases  to  be  nourished  by  its  mother 
are  in  the  teeth  and  reproductive  organs.  A  child  usually 
acquires  part  of  its  first  set  of  teeth  before  it  is  weaned 
(six  to  eighteen  months) ;  but  children  are  rarely  born  with 
all  their  milk  teeth,  and  others  do  not  acquire  any  for  more 
than  a  year.  The  second  set  of  teeth  usually  appears 
between  the  ages  of  seven  and  twelve  years,  except  the 
" wisdom"  teeth  which  generally  break  through  the  gums 
before  the  twenty-fifth  year  but  sometimes  fail  to  emerge  at 
all.  The  reproductive  organs  become  functional  in  both 
sexes  between  thirteen  and  fifteen ;  the  voice  becomes  deeper 
in  the  male  and  there  are  other  accompanying  changes. 
The  ovaries  cease  their  activities  between  the  ages  of  forty 
and  fifty,  but  the  male  reproductive  organs  may  remain 
functional  throughout  life. 


CHAPTER  XXVIII 

MAN  (Continued) 
THE  MIND  OF  MAN 

Man  excels  other  animals  in  his  combination  of  erect 
attitude,  opposable  thumb,  unusual  brain  development, 
and  powers  of  speech.  The  chief  characteristic  which  has 
enabled  him  to  outstrip  all  other  competitors  in  his  reason- 
ing power.  This  of  course  has  its  seat  in  the  nervous  sys- 
tem and  is  dependent  upon  growth  and  cell-division  for 
its  full  development.  The  nervous  system  of  man,  like 
that  of  all  vertebrates,  is  formed  by  the  turning  in  of  a 
groove  which  is  finally  pinched  off  to  form  a  tube  along  the 
dorsal  side  of  the  body  (Fig.  91).  As  development  pro- 
ceeds (Fig.  104)  tHe  wall  of  the  tube  thickens  and  becomes 
folded  in  places;  long  fibers  grow  out  from  its  nerve  cells  to 
all  parts  of  the  body  and  collect  in  bundles  to  form  nerves. 
Sense  organs  develop,  usually  on  or  near  the  outside  of  the 
body,  and  become  connected  by  nerve-cell  fibers  with  the 
central  tube.  All  the  great  nervous  structure  of  an  adult 
man  has  arisen  by  growth  and  cell-division  from  the  zygote, 
formed  when  an  egg  cell  and  spermatozoon  fused. 

In  its  fully  formed  condition  the  nervous  system  consists 
of  three  classes  of  organs:  (1)  receptors,  (2)  effectors,  and 
(3)  adjusters.  According  to  Herrick  there  are,  instead  of 
"five  senses, "  about  twenty-three  kinds  of  receptor  organs* 
in  the  human  body  which  receive  stimuli  from  outside  or 
notify  the  central  nervous  system  of  conditions  within. 

*  Separate  receptors  are  stimulated  by:  touch  and  pressure,  cold,  heat, 
pain,  chemicals,  sounds,  light,  odors,  muscle  "tonus,"  tendon  "tonus," 
hunger,  thirst,  nausea,  suffocation,  disturbances  in  circulation,  sexual 
stimuli,  distention  of  cavities  (stomach,  bladder,  etc.),  obscure  abdominal 
changes,  and  tastable  substances. 

311 


312 


GENERAL  ZOOLOGY 


Many  of  these  organs  have  great  range  of  sensibility — the 
eye  can  perceive  about  two  hundred  pure  tints,  the  average 
ear  is  able  to  discriminate  some  11,000  different  pitch 
qualities.  Effectors  are  nerve  endings  for  setting  off 
organs  which  accomplish  some  particular  work.  They 
activate  voluntary  muscles,  visceral  muscles,  or  glands. 
The  adjusters  are  groups  of  nerve  cells  and  their  branches, 
which  lie  for  the  most  part  within  the  thickened  wall  of 
the  central  nervous  tube,  or  near  it.  They  receive  notice 
of  conditions  from  the  receptors,  send  out  stimuli  to  appro- 
priate effectors,  and  make  necessary  coordinations.  A 


neuropore 


cerebr 


um 


cerebrum 


cerede//um< 
A  0,  C  0 

FIG.  110. — Development  of  the  human  brain.  A,  two  weeks  after  fertilization ; 
B,  four  weeks;  C.  during  third  month  of  foetal  life;  D,  adult  brain.  (Largely 
after  Herrick.) 

slight  external  stimulus  may  set  off  a  great  many  effectors 
and  thus  cause  a  violent  reaction,  or  a  strong  stimulus  may 
be  suppressed  and  cause  no  response — all  such  matters  are 
arranged  by  the  adjusters.  For  example,  a  pioneer  may 
see  a  human  form  on  the  horizon — if  it  is  an  Indian  with  a 
gun,  he  will  run  or  hide;  if  it  is  a  member  of  his  family,  his 
nervous  system  will  make  no  response  through  effectors  to 
the  image  received  through  his  eyes. 

Man  has  many  of  his  important  sense  organs  localized  at 
the  anterior  end  of  the  body  and  it  is  but  natural  that  the 
great  adjusters  should  be  close  at  hand  in  the  thickenings  of 
the  wall  of  the  neural  tube  which  forms  the  brain.  There 


MAN  313 

are  two  great  swellings  on  the  dorsal  surface  of  the  brain: 
the  cerebrum,  which  is  the  seat  of  " voluntary"  activity, 
consciousness,  memory,  and  reason;  and  the  cerebellum  in 
which  reside  many  of  the  centers  for  controlling  the  strength 
and  steadiness  of  muscular  activity.  There  are  other  ad- 
justor  groups  of  nerve  cells  in  the  brain,  spinal  cord,  and 
among  the  internal  organs.  These  for  the  most  part  con- 
trol simple  reflexes  in  organs  near  them. 

The  nervous  activities  of  man  are  remarkable  in  two  re- 
spects: (1)  a  vast  number  of  practically  unvarying  and 
more  or  less  automatic  reflexes  (which  greatly  exceeds  that 
of  other  animals,  except  perhaps  birds)  is  controlled  by  the 
nerve  centers  in  certain  regions  of  the  brain  outside  the 
cerebrum  and  in  the  spinal  cord;  those  concerned  with  the 
vital  functions  of  the  internal  organs  being  dominated  by 
certain  parts  of  the  brain  (medulla,  etc.),  and  those  con- 
cerned with  instinctive  reactions  of  the  skeletal  muscles 
being  largely  controlled  in  the  cord.  (2)  The  higher  men- 
tal faculties  concerned  with  variable  activities  are  developed 
as  in  no  other  animal.  The  enormous  system  of  branching 
nerve  cells  in  the  outside  layer  of  the  cerebrum  makes  pos- 
sible, not  only  the  proper  adjustment  of  the  body  to  condi- 
tions as  reported  by  effectors,  but  the  storing  of  sensory 
memories  upon  which  knowledge  and  the  power  of  reason- 
ing depend. 

At  birth  a  child  has  the  main  tracts  in  its  nervous  system 
formed,  but  is  able  during  youth  to  strengthen  or  con- 
trol its  natural  endowment  by  training.  It  may  be  natur- 
ally " bright"  in  solving  mathematical  problems,  or  "dull" 
in  languages;  awkward  or  graceful;  docile  or  stubborn. 
Youth  is  the  golden  time  for  improvement,  for  there  comes 
a  day  when  the  fiber  tracts  are  no  longer  modifiable.  As 
Herrick  puts  it,  "the  docile  period  is  past,  and  though  the 
man  may  continue  to  improve  in  the  technic  of  his  per- 
formance, he  can  no  longer  do  creative  work.  Whether 
this  process  occurs  at  the  age  of  twenty  or  eighty  years,  it 
is  the  beginning  of  senility.  And,  alas,  this  coagulation  of 


314  GENERAL  ZOOLOGY 

mental  powers  often  takes  place  so  early.  Many  a  boy's 
brains  are  curdled  and  squeezed  into  traditional  artificial 
moulds  before  he  leaves  the  grades  at  school.  We  who  seek 
to  enter  into  the  kingdom  of  knowledge  and  to  continue  to 
advance  therein  must  not  only  become  as  little  children, 
but  we  must  learn  to  continue  so." 

ORIGIN  OF  MAN 

According  to  Osborn*  man  probably  arose  during  the 
Pliocene  Period  before  the  Glacial  Epoch.  At  that  time 
there  were  no  men  in  Europe,  but  the  remains  of  a  very 
primitive  ape-man  (Pithecanthropus  erectus)  have  been  dis- 
covered in  Java  associated  with  the  porcupine,  rhinoceros, 
extinct  species  of  elephants,  and  other  animals  which  lived 
in  the  Pliocene  Period.  Java  was  at  that  time  a  part  of 
the  Asiatic  continent  and  the  primitive  men  were  therefore 
free  to  migrate  north  and  west,  as  they  apparently  did. 
The  absence  of  hair  in  man  indicates  that  he  probably  had 
his  later  evolution  in  a  hot  country.  Osborn  says:  "It  is 
possible  that  within  the  next  decade  one  or  more  of  the 
Tertiary  ancestors  of  man  may  be  discovered  in  northern 
India  among  the  foot-hills  known  as  Siwaliks.  Such  dis- 
coveries have  been  heralded,  but  none  have  thus  far  actu- 
ally been  made.  Yet  Asia  will  probably  prove  to  be  the 
center  of  the  human  race.  We  have  now  discovered  in 
southern  Asia  primitive  representatives  or  relatives  of  the 
four  existing  types  of  anthropoid  apes,  namely,  the  gibbon, 
the  orang,  the  chimpanzee,  and  the  gorilla,  and  since  the 
extinct  Indian  apes  are  related  to  those  of  Africa  and  of 
Europe,  it  appears  probable  that  southern  Asia  is  near  the 
center  of  the  evolution  of  the  higher  primates  and  that  we 
may  look  there  for  the  ancestors  not  only  of  prehuman 
stages  like  the  Trinil  race  but  of  the  higher  and  truly  hu- 
man types." 

Through  recent  scientific  work  the  history  of  man  in 

*  OSBORN,  H.  F.:  "Men  of  the  Old  Stone  Age."  1915. 


MAN 


315 


Europe  is  pretty  well  known.     At  different  times  various 
races  came  in  from  the  southeast  during  the  interglacial 


EXISTING 
APES  AND 
MAN. 


GIBBON. 
Asia. 


GLACIAL     OR 
PLEISTOCENE 
AGE. 


PLIOCENE 
AGE. 


MIOCENE 
AGE. 


OLIGOCENE. 


Primitive  Gib- 
bon of  Eu- 
rope 

(Pliohylobates). 


Earliest  'Gibbons 

of  Europe 
(Pliopithecus). 


MAN 

(Homo  sapiens). 
Asia,  Europe. 


Crd-Magnon  and 
other  races. 


More  primitive  spe- 
cies, human  and 
prehuman. 

Neanderthal  race. 
Piltdown  race. 
Heidelberg  race. 

Trinil  race 
(Pithecanthropus) . 


Unknown  Pliocene 
ancestors  of  man. 


CHIMPANZEE. 
Africa. 


Ancestral  anthro- 
poids of  Asia 


Ancestral  anthro- 
poids of  Egypt 
(Propliopithecus) . 


Primitive   anthropoids 
of  Asia  and  Europe. 


Small  monkeys 
of  Egypt. 


Unknown  ancestral  stock 
of  the  Old  World  pri- 
mates, including  man. 


FIG.  111. — Ancestral  tree  of  the  anthropoid  apes  and  of  man.  (From  Osborn's 
Men,  of  the  Old  Stone  Age.  By  special  permission  of  the  publishers,  Charles 
Scribner's  Sons.) 


periods  of  the  Pleistocene.     Europe  was  partly  covered 
with  ice  four  times  and  between  each  glaciation  there  was 


316 


GENERAL  ZOOLOGY 


a  long  warm  period  (Table  I)  when  animals  from  southern 
countries  invaded  the  land.  In.  the  deposits  left  during 
the  first  interglacial  period  eoliths,  or  unchipped  stone 
fragments,  occur  which  may  have  been  formed  by  men,  but 


TABLE    I. 

Showing     Conditions     in    Europe     during  the    Development   of  Man 
Adapted     from    Osborn  's  "Men  of  the   Old   Stone  Age  * 

Time 

Climate 

COl-O                 VTAftM 

diurnal  s 

Implements 

Human  races 

Postgtoctol 
25,000  years 

/ 

Deer,      bison,     horse, 
chamois,     ibeif 

Iron      /OOO  BC 
Bronze    /OOO  yrs. 
Pottery 
PoJishecf  stone 
5OOO  yrs. 

Carving,  painting 

Clipped  flints 
2  5.  OOO  yrs. 

Homo  sapiens 

Cro-magon  Pace 
Brain   capoctfy  I8OO  c  cm 

4  Gloc/al  Period 
25.000  years 

Reindeer,     arctic  for, 
muslfor 

3.  Interglacial 
Period 

100.  OOO  years 

} 

Bison,      horse, 
hippopotamus, 
elephant,     lion, 
rhinoceros, 
sabre  -tooth  tiger 

Neanderthal  Race 
Brain  capacity  I6OO  can 

/ 

' 

Rough  flints 
B5.000  yrs 

Piltdown    Pace 
Brain  capacity  /4OOcak 

3  Glacial  Penod 
25.000  years 

c 

Pemdeer, 
wooly   mammoth 

\ 

Hippopotamus, 

rhinoceros,   ' 

elephant, 

Heidelberg  Race 

Period 

stag,     bison, 

200.000  years 

) 

horse 

2.G/ac,a/  Period 
25.  OOO  years 

( 

Pemdeer, 
wool/  mammoth. 

(  Interglocia/ 
Period 
7S.OOO  years 

) 

Hippopotamus, 
elephant, 
rhinoceros.  / 

(Eoliths?) 

1  Glacial  Period 
25.OOO  years 

( 

Mush  ox     in 
England. 

(Trinil  race  lived  in  Jaw) 
Brain  copoc/fy  9OOcfm. 

most  archaeologists  believe  that  they  are  not  of  human 
origin.  In  the  long  second  interglacial  period  the  Heidel- 
berg race  lived  in  central  Europe.  The  men  of  this  race 


MAN  317 

(Homo  heidelbergensis)  did  not  have  a  projecting  chin  like 
later  races  and  were  primitive  in  other  features. 

During  the  third  interglacial  period  the  flint  workers 
entered  Europe  and  attained  a  considerable  degree  of  simple 
culture  before  the  last  glaciation  again  drove  them  south. 
These  men  at  first  frequented  open  camping  sites  in 
sheltered  nooks  along  the  hillsides,  where  the  bones  of  ani- 
mals slaughtered  for  food  are  found  mingled  with  their  primi- 
tive implements.  Later  when  the  climate  became  cooler 
they  often  sought  the  shelter  of  caves.  Their  implements 
were  at  first  rough  fragments  of  stone  (eoliths),  but  later 
the  stones  were  chipped  to  form  various  shapes  (palseo- 
liths).  Little  is  known  of  the  Piltdown  race,  except  that 
it  existed  in  eolithic  times,  but  the  Neanderthals  (Homo 
neanderthalensis)  left  numerous  remains  in  various  parts 
of  Europe  and  we  have  considerable  knowledge  of  their 
customs.  The  bones  of  animals  hunted  for  food  are  mixed 
with  flints  and  with  their  own  skeletons  around  the  ancient 
hearths  and  give  many  clues  to  their  habits  of  life.  These 
men  hunted  the  mammoth,  rhinoceros,  wild  horse,  bison, 
cattle,  giant  deer,  and  reindeer.  Both  flesh  and  pelts  were 
utilized,  and  the  marrow  was  sought  by  splitting  all  the 
larger  bones.  The  chase  was  pursued  with  spears  and 
darts  fitted  with  flint  points;  also  by  means  of  thro  wing- 
stones.  The  Neanderthals  may  have  used  other  means 
to  secure  large  animals,  for  their  small  spears  were  prob- 
ably not  very  effective  against  the  mammoth  or  rhinoceros. 
At  the  end  of  their  culture,  bone  came  to  be  used  somewhat 
for  anvils  and  implements.  The  Neanderthals  as  a  race 
possessed  many  structural  peculiarities — the  brows  were 
heavy  and  overhanging;  the  forehead  was  low;  nose,  flat; 
upper  lip,  long;  arms  and  shins,  short.  They  became 
extinct  during  the  fourth  glacial  period  and  were  suc- 
ceeded by  a  race  which  greatly  excelled  them  in  physique 
and  intelligence. 

The  Cro-Magnon  race,  with  others  of  less  importance, 
entered  Europe  at  the  end  of  the  fourth  glacial  period — 


318 


GENERAL  ZOOLOGY 


LOWER 
PALEOLITHIC 


\       Races  belonging 
•  £<?       B    S 


fjjbsExiifct  Stfecies 

i  t  o// 

%  I     * 


•  CL  •  I                         ^      . 

J  ***  i  I  Existing  Species  of  Man 

I  /             rj          J*        .      J 

.1  /           nomo  tsapiens 

\fces  \     \  !                    f    ? 

^"k     \l  t 

\t°     J       V  / 


Cdmtnon  Ancesi&f-s 

of 
Exiinci  and  Exisiing   Species  t>J  Man 


FIG.  112. — Tree  showing  the  main  theoretic  lines  of  descent  of  the  chief  Pre- 
Neolithic  races  discovered  in  Western  Europe.  (From  Osborn's  Men  of  the  Old 
Stone  Age.  By  special  permission  of  the  publishers,  Charles  Scribner's  Sons.) 


MAN  319 

about  25,000  years  ago.  These  men  are  placed  in  the  same 
species  (Homo  sapiens)  as  those  of  modern  times;  in  fact, 
some  are  believed  to  have  existed  until  the  fifteenth  century 
in  the  Canary  Islands.  They  used  bone  needles,  harpoons 
with  recurved  teeth,  awls,  hammers,  borers,  polishers,  and 
perhaps  bows  and  arrows.  They  also  had  considerable 
artistic  ability,  engraving  on  stone,  bone,  and  ivory; 
sculpturing  in  stone,  bone,  and  clay;  painting  on  the  walls 
of  caverns;  and  making  conventional  ornamental  figures. 
There  was  probably  bartering  at  this  time,  for  shells  and 
other  objects  from  the  Mediterranean  and  Atlantic  are 
found  in  Central  Europe. 

The  Cro-Magnon  race  declined  in  later  palaeolithic 
times,  though  some  groups  persisted,  and  their  descendants 
are  believed  to  be  now  living  in  the  region  of  Dorgodne 
and  in  some  other  places  in  France.  The  Old  Stone  Age 
(Palaeolithic)  lasted  until  10,000  B.C.;  then  four  new  races 
invaded  Europe,  probably  from  the  south  and  east.  The 
new  inhabitants  lived  more  by  fishing  than  hunting,  and 
stag-horn  harpoon  points  replace  the  older  reindeer-horn 
spear  tips  in  the  deposits  from  this  time.  Art  also  declined 
and  paintings  were  mostly  confined  to  geometric  figures 
on  pebbles,  which  may  have  had  some  religious  significance. 

In  the  Neolithic  (New  Stone)  Age  polished  stone  imple- 
ments succeeded  chipped  stone  and  agriculture  began,  for 
instruments  for  the  preparation  of  the  soil  and  the  harvest- 
ing of  crops  appear  in  the  deposits  from  this  time.  The 
most  distinctive  feature  of  the  age,  however,  was  the  intro- 
duction of  pottery.  Art  revived,  and  some  of  the  draw- 
ings show  the  dog  accompanying  man,  indicating  that  it 
had  been  domesticated.  In  the  Old  Stone  Age  the  horse 
was  commonly  eaten,  but  that  practice  died  out  as  it  came 
under  domestication.  Cattle,  sheep,  goats,  and  pigs  were 
also  domesticated  in  the  Neolithic  age.  Before  the  close 
of  this  period  all  the  direct  ancestors  of  the  modern  races 
of  Europe  had  not  only  established  themselves,  but  had 
begun  to  separate  into  colonies. 


320  GENERAL  ZOOLOGY 

During  the  Stone  Age,  "the  rudiments  of  all  modern 
economic  powers  of  man  were  developed:  the  guidance  of 
the  hand  by  the  mind,  manifested  in  his  creative  industry; 
his  inventive  faculty;  the  currency  or  spread  of  his  inven- 
tions; the  adaptations  of  means  to  ends  in  utensils,  in 
weapons,  and  in  clothing.  The  same  is  true  of  the  aesthetic 
powers,  of  close  observation,  of  the  sense  of  form,  of  pro- 
portion, of  symmetry,  and  the  appreciation  of  beauty 
of  animal  form  and  the  beauty  of  line,  color,  and  form  in 
modelling  and  sculpture.  Finally,  the  schematic  repre- 
sentation and  notation  of  ideas  so  far  as  we  can  perceive 
was  alphabetic  rather  than  pictographic.  The  religious 
sense,  the  appreciation  of  some  power  or  powers  behind  the 
great  phenomena  of  nature,  is  evidenced  in  the  reverence 
for  the  dead,  in  burials  apparently  related  to  the  future 
existence  of  the  dead,  and  especially  in  the  mysteries  of  the 
art  in  the  caverns." 

HUMAN  SOCIETY 

All  social  relations  depend  upon  mutual  toleration  and 
cooperation.  Though  those  of  man  in  part  resemble  the 
communities  of  social  insects  and  vertebrates,  which  de- 
pend largely  on  primitive  feeding  and  sheltering  instincts, 
they  have  more  of  reasonings,  elf  interest  and  altruistic 
love.  The  immediate  ancestors  of  man  were  doubtless 
social  in  habits  and  probably  possessed  the  power  of  com- 
munication by  speech.  The  men  of  the  Stone  Age  must 
have  hunted  together  for  the  cave  bear  or  mammoth  and 
cooperated  in  the  division  of  the  spoils.  They  helped  each 
other  in  averting  dangers  to  themselves  and  their  families; 
they  dwelt  together  in  the  shelter  of  caves.  Such  associa- 
tion of  different  types  of  mind  led  to  the  exchange  of  ideas 
—men  were  stimulated  to  equal  or  excel  their  companions; 
improved  methods  originated  by  one-  were  imitated  by 
another.  Members  of  the  small  .society  gained  mental 
discipline  and  self  control  because  they  were  obliged  to 
sacrifice  their  own  interests  for  those  of  the  community. 


MAN  321 

Thus  mind  and  thought  have  in  part  been  products  of  the 
social  medium. 

The  daily  association  of  man  with  man  made  possible  the 
many  inventions  and  improvements  in  methods,  tools,  and 
other  accompaniments  of  social  life.  There  was  progress 
from  the  use  of  rough  stones,  to  the  utilization  and  manu- 
facture of  chipped  stones,  polished  stones,  horn,  bone, 
bronze,  iron,  steel  and  other  metals.  Some  accidental 
burning  of  clay  about  a  hearth  perhaps  led  to  the  general 
use  of  pottery.  The  difficult  and  uncertain  capturing  of 
food  through  the  chase  gave  place  to  the  surer  method  of 
rearing  domestic  animals.  The  keeping  of  flocks  was  accom- 
panied and  in  part  superseded  by  agriculture.  As  hands 
became  more  skillful  and  minds  more  critical  the  manufac- 
ture of  merely  utilitarian  implements  was  not  satisfying— 
objects  were  made  more  symmetrical^  embellished  with 
engraving,  and  finally  art  became  an  end  in  itself.  By 
conserving  his  resources  and  making  his  food  supply  sure 
man  gained  some  leisure,  and  this  led  to  the  invention  of 
games  in  order  that  he  might  share  pleasant  experiences  and 
secure  training  in  company  with  his  fellows. 

The  collecting  of  groups  of  men  for  common  interests  and 
the  enjoyment  of  social  relations  led  to  a  "  consciousness  of 
kind. "  Each  group  drifted  into  certain  methods  and  habits 
which  became  traditions  for  their  children — thus  clans  with 
characteristic  folk  ways  arose.  Primitive  races  in  all  parts 
of  the  world  frequently  adopt  specific  marks  to  indicate 
their  clan — nose  rings,  ear  rings,  painting  the  body,  filing 
the  teeth,  flattening  the  head,  etc.  The  fostering  of  clan 
spirit  led  to  greater  affection  among  members  of  a  group 
and  also  brought  about  feelings  of  intolerance  for  other 
groups.  It  was  natural,  then,  that  there  should  be  different 
dialects  and  customs.  Certain  laws  were  adopted  by  the 
clan  and  racial  traditions  developed  so  that  group  loyalty 
in  time  gave  rise  to  patriotism.  Group  loyalty  is  strongest 
in  primitive  people;  the  most  enlightened  are  tolerant  of 
the  idiosyncrasies  of  others. 


322  GENERAL  ZOOLOGY 

The  first  human  societies  were  families.  Such  primitive 
family  groups  still  exist  among  certain  tribes  in  the  Philip- 
pine Islands  and  elsewhere.  Among  the  American  Indians 
each  tribe  consists  of  blood  relations  who  trace  their  lineage 
to  common  ancestors.  At  the  time  white  men  came  to 
America  there  were  about  20,000  Iroquois  living  in  the 
forests  of  New  York.  They  were  divided  into  separate 
"nations/5  each  consisting  of  relatives,  and  to  prevent 
close  intermarriage  there  were  laws  which  required  young 
men  to  take  wives  from  a  different  nation.  Though  these 
nations  were  affiliated  there  was  no  chief  who  ruled  them  all. 
The  feudal  chiefs  of  Bible  times  and  during  the  Middle 
Ages  were  rich  in  cattle  and  sheep.  In  addition  to  their 
relatives,  they  required  serfs  and  vassals  to  till  the  soil  and 
tend  flocks.  These  were  voluntary  retainers  who  sought 
the  protection  of  some  powerful  chief,  or  captives  taken  in 
battle.  From  affiliations  between  such  overgrown  feudal 
families  the  nations  of  the  earth  arose. 

In  the  development  of  nations  climate  has  been  an 
important  factor.  Where  poor  soil  is  coupled  with  arid 
conditions,  the  people  are  nomads  and  never  form  great 
societies.  The  struggle  with  nature  for  a  bare  living  is  too 
severe.  But  where  there  is  good  soil  and  abundant  rain- 
fall, agriculture  and  flocks  flourish.  Such  resources  enable 
men  to  acquire  permanent  homes,  accumulate  wealth,  have 
leisure  time  for  cultural  pursuits,  and  to  build  up  nations. 
On  the  other  hand  adverse  climatic  changes  have  made  an 
end  of  great  nations — the  ruins  of  ancient  Palmyra,  which 
now  lie  in  a  desert,  were  built  in  the  midst  of  agricultural 
plenty. 

Through  social  intercourse  man  has  made  progress. 
There  has  been  a  gradual  transition  from  the  slaughter  and 
capture  of  animals  or  the  collecting  of  natural  resources  in 
the  way  of  helpless  shell  fish,  fruits,  vegetables  and  seeds. 
The  chase  has  been  assisted  by  the  invention  of  traps, 
arrows,  spears,  and  firearms;  experience  has  led  to  prepara- 
tion for  times  of  scarcity  by  storing  reserves,  and  the  inven- 


MAN  323 

tion  of  methods  for  curing  food;  the  results  of  agriculture 
have  been  multiplied  by  cultivation  and  the  selection  of 
seeds.  Modern  economic  relations  have  grown  up  chiefly 
through  the  accumulation  and  exchange  of  food  products. 
Even  the  struggle  between  capital  and  labor  is  the  result 
of  primitive  instincts  to  secure,  to  store  up,  and  to  improve 
the  food.  In  modern  civilization  food  and  good  living 
depend  upon  work  or  the  possession  of  the  results  of  some- 
body's lafeor.  This  has  led  man  to  adopt  regular  habits  of 
life  in  order  to  be  more  effective  and  thus  earn  a  better 
living. 

CIVILIZATION 

The  Universal  Dictionary  says:  " Civilization  consists  in 
what  may  broadly  be  called  culture  in  a  nation;  and  a 
nation  may  be  considered  as  civilized  when  a  large  propor- 
tion of  those  belonging  to  it  have  their  intellectual  and 
moral  faculties  and  all  their  higher  nature  in  large  measure 
developed  and  becoming  increasingly  so  with  the  advance  of 
years.  Before  this  can  take  place  a  considerable  amount  of 
material  prosperity  must  have  been  achieved,  between 
which  and  the  culture  already  described  there  are  continual 
action  and  reaction.  Regarding  progression  in  material 
prosperity,  certain  stages  tend  to  occur:  (1)  a  barbarous 
one,  in  which  one  feeds  on  roots,  fruits,  and  fishes,  when 
these  last  can  be  caught  without  effort;  (2)  the  state  of  a 
hunter;  (3)  that  of  the  shepherd,  in  which,  to  avoid  the  un- 
certainty of  the  result  in  hunting,  wild  animals  are  domes- 
ticated; (4)  the  agricultural  state;  and  (5)  that  of  manu- 
factures and  commerce." 

Civilization  rests  primarily  on  commerce.  In  primitive 
society  there  is  little  or  no  exchange  of  products;  barter 
depends  upon  a  market  for  surplus  and  cannot  occur 
regularly  in  isolated  communities  where  conditions  of  life 
are  hard.  The  Esquimaux  tribes,  for  example,  are  practi- 
cally self-sustaining.  Civilized  nations  have  gone  beyond 
the  exchange  of  actual  products  and  have  money  as  a 


324  GENERAL  ZOOLOGY 

medium  of  exchange.  This  condition  is  accompanied  by 
more  or  less  segregation  into  classes  or  castes;  those  with 
limited  abilities  or  opportunities  do  the  routine  labor  of  the 
community,  those  with  unusual  endowments  or  good  for- 
tune accumulate  stores  of  desirable  assets.  There  is  thus  a 
continual  struggle  between  those  who  sell  labor  and  those 
who  have  wealth.  The  most  pitiable  " hangers-on"  of 
modern  civilization  are  those  who  make  no  contribution  to 
society,  but  live  parasitically  on  the  fruits  of  the  labor  or 
the  wealth  of  others.  Commerce  tends  to  eliminate  tradi- 
tion and  the  bigoted  following  of  the  folk  ways  of  particular 
communities.  As  one  nation  associates  with  another  and 
finds  good  workmanship,  bright  intellects,  and  shrewd 
minds,  the  contact  is  conducive  to  mutual  respect  and 
esteem. 

The  position  of  women  in  any  nation  is  a  fair  standard  of 
its  civilization.  In  primitive  societies  men  fight  and  hunt, 
women  do  all  other  necessary  work.  Work,  therefore,  is 
often  looked  down  upon  as  something  effeminate  and  un- 
worthy of  strong  manhood.  This  attitude  makes  lazy 
men  and  overburdened  women — a  strong  nation  cannot 
result.  Another  evil  that  has  often  crept  into  half-civilized 
nations  is  slavery.  In  olden  times  slaves  were  one  of  the 
recognized  rewards  of  conquerors  and  women  were  prac- 
tically slaves  to  men.  But  slavery  has  decreased  because 
it  cannot  be  successful  unless  large  quantities  of  new  land 
are  available  for  tillage.  Furthermore,  slaves  are  expensive 
and  wasteful.  They  do  not  work  to  their  limit  of  produc- 
tiveness because  they  can  receive  no  reward  except  "  self- 
maintenance."  No  highly  civilized  nation  countenances 
slavery  today.  In  modern  society,  also,  women  are  fast 
gaining  legal,  industrial,  and  intellectual  equality  with 
men.  This  does  not  mean  that  one  sex  will  or  should  be- 
come like  the  other,  but  if  civilization  progresses  each  will 
have  equal  rights  in  society. 

In  the  most  cultured  nations  we  can  see  many  evi- 
dences of  the  primitive  instincts  for  self-maintenance,  self- 


MAN  325 

protection,  and  race  preservation.  Some  dull  souls  are 
satisfied  with  mere  self-maintenance  and  strive  only  for 
enough  to  eat  and  wear.  Many  business  men  are  over- 
cautious, stingy,  and  conservative,  fearing  for  their  own 
protection;  others  are  greedy,  unscrupulous,  and  aggressive 
in  order  to  be  sure  to  have  enough.  Our  instincts  for  race 
preservation  offer  many  problems — shall  capital  criminals 
be  killed  or  encouraged  to  live  better;  shall  hopeless  idiots 
and  insane  persons  be  killed,  sterilized,  or  allowed  to  live 
and  breed  other  defectives?  Class  feeling  not  only  dis- 
plays itself  today  in  patriotism,  wars,  and  great  struggles 
for  national  industrial  supremacy,  but  is  apparent  in  many 
harmless  idiosyncrasies — such  as  cuffs  on  trousers,  starched 
collars,  the  length  of  coat  tails,  and  the  trimming  of  hats. 

In  civilized  life  there  must  always  be  adjustment  be- 
tween individual  rights  and  those  of  society.  Economic 
conditions  are  keen,  and  the  cities,  which  are  the  greatest 
industrial  centers,  show  the  greatest  extremes — wealth, 
leisure,  and  culture  contrasting  with  long  hours,  poverty, 
overcrowding,  and  ignorance.  In  civilization  there  must 
always  be  cooperation,  sacrifice,  and  delegated  authority. 
Often  it  seems  that  justice  is  obsolete,  if  individual  cases 
are  considered,  but  on  the  whole  man's  civilization  grows 
higher  and  better.  Certain  aspects  of  modern  life  are 
highly  desirable — the  free  interchange  of  ideas,  the  de- 
pendence of  industrial  progress  on  scientific  method  and 
discovery,  the  intellectual  opportunities  offered  to  all. 
But  other  sides  of  this  life  are  not  so  desirable — the  growing 
importance  of  labor-saving  machines  leads  man  more  and 
more  indoors  where  he  falls  prey  to  the  diseases  accom- 
panying sedentary  life;  abnormal  instincts  for  self-main- 
tenance lead  to  the  hoarding  of  vast  fortunes  which  at  times 
menace  society;  the  wisdom  of  expending  lives  and  resources 
in  warfare  is  certainly  questionable. 

Religious  beliefs  of  some  sort  have  accompanied  man's 
culture  since  its  beginnings  in  the  Stone  Age.  Many  of 
the  great  reforms  in  society  have  been  brought  about  by 


326  GENERAL  ZOOLOGY 

changing  religious  ideals,  and  in  this  field  Christianity  has 
done  more  for  improvement  than  any  other  religion. 
Most  primitive  religious  beliefs  have  arisen  from  a  desire 
for  present  help,  retribution,  or  future  life,  and  some  races 
have  been  particularly  "  susceptible "  to  religion.  The 
negroes,  for  example,  in  a  savage  state  commonly  believe 
in  magic,  charms,  and  the  like;  and  when  civilized  are  often 
religious  enthusiasts.  Semicivilized  people  generally  be- 
lieve in  totems,  charms,  and  transmigration.  For  example, 
the  Alaskan  Indian  thinks  that  by  making  a  clay  image  of 
his  enemy  and  destroying  it,  he  has  injured  the  enemy;  and 
that  by  wearing  a  charm  simulating  some  animal  he  acquires 
certain  of  the  qualities  of  that  animal.  What  the  outcome 
of  the  " higher  criticism"  of  modern  religion  will  be  is  un- 
certain, but  one  thing  is  sure — no  religion  which  is  largely 
form  without  high  ideals  and  " service"  can  survive. 

Probably  the  greatest  burden  that  civilization  and  knowl- 
edge lays  on  man  is — responsibility.  If  man  rules  the  earth, 
he  must  do  it  with  wisdom,  kindness,  and  justice.  He  must 
conserve  natural  resources;  protect  helpless  and  ignorant 
nations.  Through  improvement  in  educational  methods  he 
must  find  some  means  to  make  the  great  stores  of  knowledge 
available  as  easily  as  possible.  Education  must  more  and 
more  pass  from  mere  training  in  observing  and  memorizing 
to  training  for  the  exercising  of  powers  of  generalization  and 
ingenuity.  Education  has  as  its  object  the  power  to  con- 
trol self — to  do  more  work  and  to  do  it  better.  Each 
generation  must  excel  the  one  before  and  we  do  not  know 
what  the  future  of  man  will  be.  If  he  goes  ahead  as  he  has, 
it  is  possible  that  finally  the  universal  exchange  of  ideas 
may  bring  about  a  ripe  culture — which  will  mean  universal 
cooperation,  sacrifice,  and  toleration.  If,  however,  a  great 
disaster  wipes  out  the  human  race,  some  other  animal  will 
have  to  struggle  up  through  another  long  evolution  and  take 
its  place.  In  either  event  the  manifest  duty  of  man  is  to 
do  his  best — to  struggle  continually  to  improve.  Great 
nations  must  have  great  minds  and  spirits;  these  have 
always  come  from  people  who  work. 


CHAPTER  XXIX 
ANIMALS  OF  THE  PAST 

Chamberlin*  maintains  that  the  earth  could  not  have 
originated  from  the  gradual  condensation  of  a  gaseous 
nebula,  as  Laplace  believed,  but  was  probably  ejected  from 
the  sun  on  account  of  the  attraction  exerted  by  a  passing 
star.  He  believes  that  it  probably  solidified  very  soon  and 
that  it  grew  somewhat  by  the  accumulation  of  smaller 
bodies  (planetesmals)  on  its  surface.  "The  juvenile  shap- 
ing of  the  earth  may  be  said  to  have  begun  as  soon  as  the 
planetesmals  began  to  plunge  into  the  earth-knot  of  the 
nebula;  and  both  knot  and  planetesmals  began  to  gather  into 
a  dense  body.  The  drawing  of  an  atmosphere  close  about 
the  young  earth  commenced  almost  simultaneously.  The 
gathering  of  the  primitive  waters  into  the  hollows  of  the 
earth-surface  soon  followed.  -These  three  concurrent  activi- 
ties were  master-processes  in  the  growth  of  the  infantile 
earth;  they  were  the  geologic  triumvirate.  They  wrought 
together  toward  the  earth's  final  shaping  into  the  litho- 
sphere,  the  hydrosphere,  and  the  atmosphere.  The  con- 
tact surfaces  between  earth,  air  and  water  are  the  sites  of 
the  most  distinctive  activities  of  the  present  day,  and  as 
far  back  as  a  good  record  goes  these  contact  zones  have  been 
the  seats  of  the  most  declared  denudations  and  depositions. 
They  have  been  almost  the  sole  habitats  of  biological  and 
psychological  activity.  From  the  naturalistic  point  of 
view  these  climatic  developments  embody  three  great  steps : 
(1)  an  ascent  in  the  complexity  of  physico-chemical  combi- 
nation until  it  attained  the  organic  type;  (2)  an  evolution 
of  physiological  processes  and  of  organs  subservient  to 
these;  and  (3)  the  initiation  and  the  varied  development  of 

*  "The  Origin  of  the  Earth,"  1916. 

327 


328 


GENERAL  ZOOLOGY 


Fig,  lid.  Geological  History 


Pertod 


Characteristic  Animals 


Firtt 

Occurrence  of 


Recent 


P/e/sTocene 


Mammofh. 

Horse. 
GtcjphodonTs. 


Man 


Deer  Sloths, 
Ape . 


Dog  .  Stag. 
Came/, 

Ape  .  Marfl 


Miocene 


Elephant. 
Sab re. '-tooth 


Car.     Bear, 

Monkey, 

Cow ,  Oeer. 


Mgocene 


hoofed 


^Lr-^j,^ 


Horse  (3  toes) ^ 
Rhinoceros. 
Pike 


Eocene 


Mammals. 


Snake.  .Lemur 
BaT.    hog. 
horse. 


Cretaceous 


Mars  up  ia  Is , 
Salamanders 


Jurrass/c 


Bird. 
Crocodile. 
Frog. 


Reptiles  . 
Amphibians, 


Mammals. 

Turtles. 
Dinosaurs. 


Reptiles 


BryO7.oan$. 
Echinolds. 
Ophiurotds . 


Cambrian 


"rustacea  ns, 
Molluscs. 
Worms .  etc. 


Qracniopods, 
Tr/lotfifeS. 


rocks  -    no  f 053 Us 


FIG.  113. 


ANIMALS  OF  THE  PAST  329 

psychological  phenomena.     These  seem  to  have  followed 
one  another  in  ascensive  order." 

Though  Chamberlin  is  willing,  on  the  basis  of  observed 
phenomena  and  mathematical  calculations,  to  postulate 
that  life  originated  in  the  surface  layers  of  the  soil  near 
water,  there  is  no  actual  record  of  such  origin.  The  life  of 
the  earliest  geological  periods  has  left  no  trace  because  the 
rocks  have  all  been  "metamorphosed/'  that  is,  changed  by 
pressure,  heat,  or  other  influences  so  as  to  destroy  all 
organic  remains.  Though  Walcott  has  recently  discovered 
a  few  fossils  in  Pre-cambrian<rocks,  the  sedimentary  depos- 
its in  the  Cambrian  are  the  first  which  contain  abundant 
animal  remains.  In  the  Cambrian  most  of  the  chief  groups 
of  invertebrate  animals  appeared  and  many  of  them  had 
already  attained  a  high  degree  of  specialization  during  the 
preceding  formative  ages.  Fig.  113  gives  a  condensed 
summary  of  the  chief  events  since  Pre-cambrian  times. 
The  characteristic  animals  of  the  four  great  geological  ages 
will  now  be  considered. 

PALAEOZOIC  AGE 

Cambrian  Period. — The  Cambrian  fossils,  so  far  as  the 
animals  are  concerned,  are  all  of  marine  origin,  and  it  is 
doubtful  if  -terrestrial  animals  existed.  In  the  oceans  there 
were  protozoans,  sponges,  hydrozoans,  jelly-fishes,  corals, 
round-worms,  annelids,  echinoderms  (cystoids,  starfishes, 
sea  cucumbers),  brachiopods,  clams,  snails,  cephalopods, 
crustaceans  and  arachnids.  Doubtless  other  types  of  inver- 
tebrates were  in  existence  but  their  remains  have  not  yet 
been  found.  Some  of  the  species  which  existed  in  Cam- 
brian oceans  did  not  survive  even  until  the  next  period, 
and  many  whole  groups  became  extinct  during  the  Palae- 
ozoic Age.  Trilobites  of  various  sizes  were  common. 
The  cephalopods  usually  had  the  body  enclosed  in  a  straight 
or  slightly  curved  shell. 

Ordovician  Period. — In  the  rocks  from  this  period  appear 
the  first  remains  of  several  classes  of  echinoderms  (Bias- 


330  GENERAL  ZOOLOGY 

toidea,  Crinoidea,  Ophiuroidea,  Echinoidea),  the  Bryozoa, 
and  certain  molluscs.  The  trilobites  attained  their  cli- 
max here  and  declined  during  succeeding  periods.  The 
cephalopods  began  to  have  coiled  shells,  which  were  less 
cumbersome  than  the  straight  types  characteristic  of  the 
Cambrian.  The  graptolites,  a  peculiar  group  of  ccelen- 
terates,  flourished  in  this  time,  but  had  a  short  racial  exist- 
ence for  they  were  practically  extinct  at  the  end  of  the 
next  period. 

Silurian  Period. — The  most  notable  event  recorded  in 
Silurian  rocks  is  the  advent  of  fish-like  animals.  The 
ostracoderms  had  no  paired  fins  or  true  jaws  and  differed 
from  modern  fishes  in  many  other  respects.  Many  of 
them  had  the  anterior  end  of  the  body  covered  with  heavy 
protective  plates.  There  were  also  primitive  sharks  in  the 
Silurian  seas.  In  this  period  also  the  corals  developed  suf- 
ficient colonial  life  to  begin  to  construct  reefs;  the  brachio- 
pods  reached  the  height  of  their  racial  vigor;  crinoids  be- 
came abundant;  the  trilobites  persisted  in  considerable 
numbers;  the  king-crabs  had  attained  some  specialization, 
and  primitive  sea-scorpions  were  in  existence. 

Devonian  Period. — Fish-like  animals  increased  greatly 
and  many  new  types  appeared.  Though  ostracoderms  per- 
sisted and  the  appearance  of  new  forms  indicates  that  they 
attained  considerable  specialization,  the  whole  group  died 
out  at  the  end  of  the  period.  Cyclostomes  and  sharks 
were  present,  and  some  of  the  latter  attained  a  length  of 
five  feet.  Several  types  of  true  fishes  were  in  evidence- 
some  were  apparently  ancestral  to  those  living  today  and 
some  have  become  extinct.  The  arthrodirans  had  the 
anterior  part  of  the  body  covered  with  heavy  exoskeletal 
plates,  and  were  unique  in  possessing  a  neck  joint  so  that 
they  could  nod  the  head.  The  trilobites  were  now  declin- 
ing and  were  replaced  by  crustaceans  of  more  modern  type. 
Most  of  the  cephalopods  possessed  coiled  shells,  which  in 
many  species  had  a  very  complicated  structure;  clams  be- 
came specialized  to  such  an  extent  that  forms  much  like 


ANIMALS  OF  THE  PAST  331 

modern  oysters  and  razor-clams  were  in  existence.  The 
presence  of  primitive  myriapods  indicates  that  the  land  had 
a  well-established  fauna  at  this  time. 

Carboniferous  Period. — The  rocks  of  the  coal  measures 
are  of  interest  chiefly  because  they  contain  the  remains  of 
the  first  known  animals  with  pentadactyl  limbs.  These 
were  amphibians  and  some  of  them  at  least  breathed  air 
into  the  lungs  when  in  the  adult  condition.  They  lived  in 
fresh  water  and  some  may  have  been  more  or  less  terres- 
trial. All  the  Carboniferous  amphibians  are  called  stego- 
cephalians  because  they  had  the  head  "  roof ed-over "  with 
bony  plates.  Some  were  several  feet  in  length  and  a  num- 
ber had  teeth  of  very  complicated  structure  (labyrintho- 
donts).  Sharks  were  numerous  and  widespread  at  this 
time;  true  fishes  were  increasing  in  numbers. 

Toward  the  end  of  the  Carboniferous  Period  insects  re- 
lated to  orthopterans,  may-flies,  and  dragon-flies  were  living 
in  the  land  vegetation,  and  there  were  other  types  which 
later  became  extinct.  Various  arachnids  and  a  number  of 
species  of  myriapods  were  also  living  on  land.  The  brachi- 
opods  and  the  older  classes  of  molluscs  and  echinoderms 
were  declining  in  numbers.  Only  a  single  family  of  trilo- 
bites  persisted  to  the  end  of  this  period.  The  abundant 
land  vegetation  was  largely  responsible  for  the  coal  deposits 
that  are  being  mined  at  the  present  time. 

Permian  Period. — The  number  of  species  of  amphibians 
increased  somewhat  during  this  period,  but  the  most  inter- 
esting feature  is  the  rise  of  the  reptiles,  which  came  into 
existence  in  the  preceding  period  and  now  invaded  aquatic 
and  terrestrial  habitats.  Reptiles  soon  showed  specializa- 
tion along  a  number  of  different  lines.  Some  Anomodontia 
resembled  mammals  in  many  structural  features,  and  in  all 
probability  the  mammals  later  arose  from  this  stock. 
Many  of  the  old  types  of  fishes  were  dying  out,  even  that 
which  probably  gave  origin  to  the  amphibians  declined, 
but  their  places  were  taken  by  species  more  like  those  of 
modern  times.  The  trilobites,  so  characteristic  of  ancient 


332  GENERAL  ZOOLOGY 

seas,  became  extinct  and  the  ancestors  of  decapod  crusta- 
ceans appeared.  The  land  vegetation  consisted  largely  of 
coniferous  trees. 

At  the  beginning  of  the  Palaeozoic  Age,  animals  were  rep- 
resented only  by  invertebrates,  but  toward  its  close  fish- 
like  chor dates,  amphibians  and  reptiles  arose,  and  gained 
prominent  places.  At  the  beginning  also,  most  of  the 
animals  were  marine,  but,  though  the  ocean  continued  to 
be  abundantly  populated,  as  life  progressed  in  specialization 
there  was  an  increasing  invasion  of  fresh-water  and  land 
habitats.  Many  groups  that  were  important  in  Cambrian 
times  had  dwindled  (Brachiopoda)  or  become  extinct  (Tri- 
lobita)  at  the  close  of  the  Permian. 

MESOZOIC   AGE 

Triassic  Period. — During  Triassic  times  the  land  was 
covered  by  great  forests  of  coniferous  trees,  mixed  with  the 
older  club  mosses,  ferns,  and  algae.  This  luxuriant  growth 
supported  numerous  insects  and  such  modern  types  as  the 
beetles  had  made  their  appearance.  Some  of  the  stego- 
cephalians  at  this  time  were  of  great  size,  one  species  hav- 
ing a  skull  measuring  more  than  a  yard  in  length.  Reptiles 
were  more  diverse  and  attained  greater  size  than  during  the 
preceding  period.  The  giant  dinosuars  had  appeared  in 
various  places;  heavy-bodied  plesiosaurs  were  beginning 
to  frequent  the  shores  of  oceans;  whale-like  ichthiosaurs 
competed  with  fishes  in  marine  habitats;  chelonians  fre- 
quented the  swamps  and  rivers.  Small  mammals  also  ap- 
peared in  the  Triassic  period,  but  did  not  take  an  important 
place  among  animals  until  later. 

Jurassic  Period. — Reptiles  rose  to  the  zenith  of  their 
development  in  this  period  and  were  distributed  in  all  parts 
of  the  earth.  Herbivorous  and  carnivorous  dinosaurs 
(Fig.  101)  dominated  the  land  and  frequented  the  borders 
of  marshes;  tortoises  were  numerous  along  rivers;  the  ocean 


ANIMALS  OF  THE  PAST  333 

was  frequented  by  large  plesiosaurs  and  ichthiosaurs;  the 
air  served  as  a  highway  for  the  ''flying  lizards,"  or  ptero- 
dactyls (Fig.  101).  In  later  Jurassic  times  primitive  birds 
(Fig.  102)  appeared,  and  for  a  time  shared  the  ah-  with  the 
flying  reptiles.  Mammals  continued  to  be  small  in  size, 
but  had  differentiated  into  types  suggesting  moles,  opos- 
sums, and  rodents.  Fishes  began  to  have  more  and  more 
the  structures  associated  with  those  of  modern  times:  the 
endoskeleton  became  less  cartilaginous  and  more  bony;  the 
heavy  armour  on  the  outside  of  the  body  decreased;  the 
fins  became  more  effective  organs  for  swimming. 

Cretaceous  Period. — Dinosaurs  and  other  great  reptiles 
still  flourished  at  the  beginning  of  this  period  but  before 
the  close  many  whole  groups  had  died  out  completely. 
The  gigantic  size  of  some  of  these  animals  is  indicated  by 
Shimer's  estimates  concerning  Brontosaurus,  an  herbi- 
vorous dinosaur,  which  is  believed  to  have  weighed  twenty 
tons  and  to  have  consumed  about  4000  pounds  of  leaves 
and  twigs  per  day.  The  birds  of  this  time  were  not  lizard 
like.  Their  tails  were  short  and  the  wings  did  not  generally 
possess  free  digits,  but  the  jaws  were  armed  with  teeth. 
In  some  types  there  were  signs  of  extreme  specialization. 
Hesperornis,  for  example,  had  lost  its  power  of  flight  and 
the  wings  were  degenerate.  The  mammals  were  still 
small  and  for  the  most  part  not  highly  specialized. 

During  the  Mesozoic  Age  the  reptiles  were  the  most  im- 
portant animals;  though  birds  and  mammals  came  into 
existence  they  did  not  play  a  prominent  part  in  the  life  of 
the  time.  Toward  the  close  of  the  age  many  orders  of 
reptiles  became  extinct.  Pterodactyls  no  longer  sailed 
through  the  air;  ichiosaurs  and  plesiosaurs  were  absent 
from  the  ocean;  dinosaurs  had  disappeared  forever. 
Among  invertebrate  animals  there  had  also  been  many 
changes.  In  the  ocean  true  shrimps,  lobsters,  and  crabs 
had  come  into  existence;  the  archaic  cephalopods,  so 
characteristic  of  ancient  seas,  were  largely  replaced  by  more 


334  GENERAL  ZOOLOGY 

modern  devil-fishes.  On  land  insects  were  abundant  and 
had  probably  come  to  take  some  part  in  the  pollination  of 
plants. 

TERTIARY  AGE 

Eocene  Period. — There  were  great  changes  of  scene  be- 
tween the  Cretaceous  and  Eocene.  Many  old  types  of 
reptiles  and  other  animals  were  extinct;  mammals  now 
became  the  most  important  land  animals.  The  ancestral 
stocks  of  rodents,  elephants,  bats,  ruminants,  whales, 
carnivores,  edentates,  and  insectivores  had  already  reached 
some  degree  of  differentiation.  There  were  also  other 
primitive  mammalian  strains  in  existence  which  have  since 
become  extinct.  Birds  without  teeth  were  present  in 
considerable  numbers  and  variety.  The  reptiles,  amphi- 
bians, and  fishes  all  had  a  more  modern  aspect.  Such  types 
as  lizards,  snakes,  tailless  amphibians,  and  herrings  were 
prevalent.  Among  the  invertebrates  also  there  were  hermit 
crabs,  and  other  specialized  modern  types. 

Oligocene  Period. — There  were  now  a  number  of  types 
of  hoofed  animals  in  existence.  Among  the  survivors  of 
the  strange  herbivores  of  Eocene  times,  the  rhinoceros- 
like  titanotheres  were  perhaps  the  most  remarkable,  but 
they  were  all  extinct  before  the  beginning  of  the  next  period. 
The  true  rhinoceros  stock  was  as  yet  hornless;  the  horses 
had  increased  to  the  size  of  foxes  and  their  feet  showed  some 
degree  of  specialization  toward  the  reduction  of  toes;  pigs 
were  small  creatures  with  four  usable  toes  on  each  foot;  the 
ruminants  were  without  horns,  even  the  deer.  Among  the 
carnivores  the  sabre-tooth  cats  were  perhaps  the  most 
striking  though  they  had  not  yet  attained  their  greatest  size. 
These  animals  had  the  upper  canines  greatly  elongated. 
Civet  cats  and  carnivores  related  to  modern  weasels  were 
in  existence.  Primate  remains  have  also  been  found  in 
the  deposits  from  this  period.  The  first  pioneers  of  the 
Primates  were  primitive  lemur-like  animals.  Among  the 
birds  there  were  divers  like  modern  grebes,  cormorants, 


ANIMALS  OF  THE  PAST  335 

gulls,  pelicans,  rails,  ducks,  plovers,  grouse,  parrots  and 
eagles.  The  insects  were  represented  by  such  specialized 
forms  as  gall-flies,  saw-flies,  bees,  stag  beetles,  and 
butterflies. 

Miocene  Period. — The  landscape  now  had  a  more  modern 
aspect.  Among  the  plants  were  irises,  pinks,  poppies, 
violets,  roses,  plums,  and  pineapples.  Various  types  of 
primitive  monkeys  were  living  in  the  trees.  Porcupines, 
beavers,  martens,  skunks,  bears,  antlered  deer,  seals  and 
other  modern  types  lived  with  archaic  forms  like  the  horn- 
less rhinoceri,  and  three-toed  horses.  In  South  America 
there  were  great  ground  sloths,  some  as  much  as  twenty  feet 
long,  armadillos,  and  strange  hoofed  animals.  A  number 
of  new  types  of  birds  appeared  during  Miocene  times  and 
other  vertebrates  were  also  changing.  Snakes  were  now 
represented  by  venomous  species. 

Pliocene  Period. — In  this  period  many  of  the  older 
mammalian  types  were  still  in  existence.  There  were 
long-jawed  elephants;  horses  in  which  the  outside  toes, 
though  off  the  ground,  were  still  quite  large,  sabre-tooth 
tigers,  and  other  archaic  creatures.  But  such  old-fashioned 
animals  were  outnumbered  by  the  modern  types  and  old 
strains  continued  to  die  out.  At  the  end  of  the  period  there 
were  a  number  of  kinds  of  true  apes  scrambling  about  in 
the  trees;  the  horses  were  mostly  " one-toed ;"  and  there 
were  many  new  animals,  such  as  goats,  giraffes,  and  even 
apes  somewhat  closely  related  to  man. 

The  Tertiary  Age  is  noteworthy  for  the  increase  of  the 
warm-blooded  animals,  particularly  the  mammals.  Though 
many  lines  of  amphibians  and  reptiles  underwent  great 
specializations,  these  groups  in  general  decreased,  both  in 
numbers  and  in  variety.  The  birds  and  mammals,  .on 
the  contrary,  showed  specialization  along  many  and  various 
lines,  increased  greatly  in  numbers,  and  filled  many  of  the 
habitats  occupied  during  the  later  Mesozoic  times  by  their 
cold-blooded  predecessors.  At  the  close  of  the  Pliocene, 


336  GENERAL  ZOOLOGY 

though  there  were  many  relics  from  previous  ages,  the  great 
majority  of  animals  were  much  like  those  of  today. 

QUATERNARY  AGE 

Pleistocene  Period. — This  was  a  time  of  great  temperature 
changes,  and  the  northern  hemisphere  was  invaded  four 
times  by  great  masses  of  ice  which  came  from  the  north. 
Climatic  conditions  caused  great  migrations  among  animals. 
Europe,  for  example  was  at  times  inhabited  by  reindeer, 
arctic  foxes,  musk-oxen,  woolly  rhinoceri,  mammoths,  and 
other  animals  characteristic  of  cool  regions.  When  the  ice 
receded,  horses,  elephants,  sabre-tooth  tigers,  lions,  hippo- 
potami, and  other  animals  came  in.  During  the  warm 
interglacial  periods  several  races  of  men  dwelt  in  Europe 
and  attained  a  considerable  degree  of  primitive  culture. 
In  South  America  giant  sloths,  peculiar  camel-like  creatures, 
and  gigantic  armadillos  (Fig.  113)  were  still  in  existence. 

Since  the  last  great  glaciation  the  northern  hemisphere 
has  enjoyed  a  fairly  stable  climate,  vegetation  has  spread 
northward,  and  animals  have  increased  in  numbers.  The 
most  important  feature  of  the  Quaternary  Age  has  been 
the  rise  of  man  and  his  domination  of  the  earth. 

PAL^ONTOLOGICAL  HISTORY  OF  ELEPHANT  AND  HORSE 

The  discussion  of  the  geological  periods  necessarily  has 
been  very  limited  and  noted  only  a  few  of  the  important 
events  of  palseontological  history.  In  order  to  give  a  more 
concrete  idea  of  how  modern  types  of  animals  have  de- 
veloped from  primitive  ancestors,  two  of  the  best  known 
series  will  be  briefly  described.  These  relate  to  the  ances- 
tors of  the  elephant  and  the  horse. 

The  earliest  elephant  remains  (Fig.  114,  A)  were  dis- 
covered in  Middle  Eocene  deposits  in  Egypt.  The  crea- 
tures from  which  modern  types  probably  descended  were 
small  tapir-like  animals,  about  three  and  a  half  feet  high, 
which  appear  to  have  dwelt  in  swamps,  living  upon  the 
succulent,  semi-aquatic  herbage  of  that  time.  Their  front 


ANIMALS  OF  THE  PAST 


337 


teeth  were  rather  degenerate,  indicating  a  prehensile  upper 
lip,  and  the  second  pair  of  incisors  in  each  jaw  were  some- 
what developed  as  tusks.  In  the  Upper  Eocene  this 


FIG.   114. — Showing  fossil  ancestors  and  living  representatives  of  elephant  and 
horse.     ( After  Lull.) 

primitive  elephant  (Mceritherium)  was  asssociated  with  a 
more  specialized  type  (Palceomastodon,  B)  of  somewhat 
larger  size.  This  new  elephant  had  a  greatly  elongated 


338  GENERAL  ZOOLOGY 

lower  jaw  and  the  structure  of  its  upper  jaw  indicates  that 
it  possessed  a  short  extensile  proboscis.  No  elephant  bones 
are  known  from  Oligocene  deposits,  but  in  the  Miocene  of 
France  a  great  probocidian,  Tetrabelodon,  has  been  dis- 
covered which  is  remarkable  for  the  great  elongation  of  the 
lower  jaw  and  the  shortness  of  the  lower  tusks  (C).  In 
Pliocene  times  several  species  of  the  genus  Dibelodon  lived 
in  South  America.  These  animals  had  well-developed 
tusks  in  the  upper  jaw  but  the  short  lower  jaw  was  without 
incisors.  The  mastodon  (D),  which  lived  in  North  America 
during  glacial  times,  had  the  lower  jaw  still  further  shortened. 
Modern  elephants  (E)  apparently  took  their  origin  from 
a  straight  tusked  type,  Stegodon,  which  developed  in 
the  Indian  region.  The  genera  mentioned  are  only  a  few 
representatives  of  numerous  types  of  fossil  proboscidians. 
At  one  time  there  were  elephants  in  Europe,  Africa,  Asia, 
North  and  South  America.  From  what  is  known  Lull  is 
able  to  make  the  following  generalizations  concerning  the 
racial  history  of  the  Proboscidia: 

"The  physical  changes  undergone  by  the  race  are  also 
clearly  shown,  as  the  palseontological  series  is  very  complete. 
These  changes  may  be  thus  summarized:  Increase  in  size 
and  in  the  development  of  pillar-like  limbs  to  support  the 
enormous  weight.  Increase  in  size  and  complexity  of  teeth 
and  their  consequent  diminution  in  numbers  and  the  de- 
velopment of  the  peculiar  method  of  tooth  succession.  The 
loss  of  the  canines  and  of  all  the  incisor  teeth  except  the 
second  pair  in  the  upper  and  lower  jaws  and  the  develop- 
ment of  these  as  tusks.  The  gradual  elongation  of  the 
symphysis  or  union  of  the  lower  jaws  to  strengthen  and 
support  the  lower  tusks  while  digging,  culminating  in 
Tetrabelodon.  The  apparently  sudden  shortening  of  this 
symphysis  following  the  loss  of  the  lower  tusks  and  the 
compensating  increase  in  size  and  the  change  in  curvature 
in  those  of  the  upper  jaw.  The  increase  in  bulk  and  height, 
together  with  the  shortening  of  the  neck  necessitated  by  the 
increasing  weight  of  the  head  with  its  great  battery  of 


ANIMALS  OF  THE  PAST  339 

tusks,  necessitated  the  development  of  a  prehensile  upper  lip 
which  gradually  evolved  into  a  proboscis  for  food  gathering. 
The  elongation  of  the  lower  jaw  implies  a  similar  elongation 
of  this  proboscis  in  order  that  the  latter  may  reach  beyond 
the  tusks.  The  trunk  did  not,  however,  reach  maximum 
utility  until  the  shortening  jaw,  removing  the  support 
from  beneath,  left  it  pendant  as  in  the  living  elephant. 
The  change  in  the  form  of  the  skull  developed  pari  passu 
with  the  growth  of  the  tusks  and  trunk  as  it  is  merely  a 
mechanical  adaptation  to  give  greater  leverage  in  the 
wielding  of  these  organs.  It  may  readily  be  seen  that  these 
changes  curiously  interact  upon  one  another;  the  result  of 
the  evolution  of  its  parts  being  the  development  of  a  most 
marvelous  whole." 

Though  elephants  passed  most  of  their  evolutionary 
history  in  Africa,  America  can  claim  the  distinction  of 
having  been  the  chief  field  for  the  evolution  of  horses. 
Lull  says:  " These  horse-like  animals,  to  which  the  name  of 
hyracotheres  has  been  given  ranged  throughout  the  entire 
Eocene  period.  They  were  of  small  size  ranging  upward 
from  eleven  to  fourteen  inches  at  the  shoulder.  The  hand 
bore  four  digits,  while  in  the  foot  but  three  out  of  the 
primal  five  remained.  Hyracotherium  itself,  the  oldest  of 
its  race,  is  known  from  a  skull  found  in  the  London  Clay, 
while  both  in  England  and  America  is  found  the  widespread 
Dawn  Horse,  Eohippus.  The  scene  of  the  evolutionary 
drama  being  shifted  to  our  land,  we  find  Eohippus  (Fig. 
114,  F)  succeeded  by  the  somewhat  larger  Orohippus,  the 
mountain  horse  (G),  while  Epihippus  carries  the  race  close 
to  the  close  of  Eocene  time,  a  duration  of  at  least  two  million 
years."  During  the  Oligocene lived  Anchiiherium  with  three 
functional  toes  on  each  of  its  feet;  the  speedy  Mesohippus 
(H)  was  eighteen  inches  in  height  at  the  withers;  and  its 
successor,  Miohippus  (I),  stood  twenty-four  inches  high. 
"The  Miocene  period  was  the  millenium  of  mammals,  and 
this  was  superlatively  true  of  the  horses."  Though  three- 
toed  horses  lived  in  the  early  part  of  this  period,  they  were 


340  GENERAL  ZOOLOGY 

later  replaced  by  one-toed  types,  and  in  the  Pliocene 
appeared  the  genus  Equus  (J,  K)  to  which  all  living  horses, 
asses,  and  zebras  belong. 

GENERAL  REMARKS  ON  PAL^OZOOLOGY 

The  geological  periods  and  the  important  features  of 
palseozoology  are  shown  in  Fig.  113.  There  is  no  doubt 
that  the  great  landmarks  of  this  history  have  been  correctly 
interpreted — strata  of  sedimentary  deposits  have  been 
formed  on  the  earth's  crust  and  the  deepest  of  these  are 
the  oldest.  From  studies  of  the  characteristics  of  rocks 
and  the  animal  remains  in  such  deposits  the  following  gen- 
eralizations may  be  made : 

1.  Conditions  on  the  earth  have  varied.     There  have  been 
times  of  rapid  change  when  whole  groups  of  animals  became 
extinct  because  they  were  unsuited  to  new  conditions,  and 
these  have  alternated  with  long   periods  of   comparative 
stability  when  particular  types  of  animal  life  multiplied 
and  became  specialized  along  various  lines. 

2.  The  animals  of  the  earliest  times  were  predominantly 
marine  and  in  later  ages  the  land  and  fresh  water  have  had 
an  increasing  population.     Structure  is  so  correlated  with 
habits  of  life,  that,  taken  with  the  nature  of  the  sediment 
in  which  fossils  are  found,  it  enables  a  palaeontologist  to 
picture  the  conditions  in  which  an  animal  lived  millions 
of  years  ago. 

3.  The  geographic  distribution  of  animals  today  is  corre- 
lated with  their  origin  and  distribution  in  the  past.     A  group 
of  limited  range  is  usually  of  rather  recent  origin,  whereas 
one  which  is  widely  distributed  is  old.     Zittel  says,  "An 
understanding  of  the  physical  conditions  which  have  gov- 
erned the  perpetuation  of  recent  plants  and  animals  in  their 
respective  provinces  would  be  utterly  impossible  without  a 
knowledge  of  their  distribution  in  former  times." 

4.  The   simplest,  or  most  primitive,  animals   came  first. 
This  is  equally  applicable  to  the  sequence  in  the  develop- 


ANIMALS  OF  THE  PAST  341 

ment  of  the  different  phyla  and  to  specialization  within 
particular  groups — Protozoa  must  have  preceded  Metazoa; 
crustaceans  existed  before  insects.  Fishes,  amphibians, 
reptiles,  birds  and  mammals  have  arisen  since  Pre-cambrian 
times  and  the  order  of  their  appearance  agrees  roughly 
with  their  degree  of  specialization.  What  has  been  said 
of  animals  applies  equally  well  to  plants.  There  has  been 
a  succession  of  algae,  mosses,  ferns,  conifers,  and  finally  the 
seed-plants  of  today. 

There  have  been  great  changes;  the  types  of  animals 
living  today  are  few  compared  with  those  which  have  passed 
away  forever.  The  animal  kindgom  past  and  present  may 
be  compared  to  a  cotton-wood  tree  which  is  being  covered 
by  a  moving  dune;  the  living  twigs  are  but  a  small  part  of 
the  portions  now  covered  by  the  shifting  sands  of  time  or 
dune. 


CHAPTER  XXX 
EVOLUTION  AND  HEREDITY 

"It  has  been  a  Proteus  from  the  first;  changefulness  is  its  most  abiding 
quality;  in  short,  the  essence  of  the  creature  is  its  innate  creativeness." 
THOMSON  AND  GEDDES. 

In  the  past  various  theories  have  been  held  in  regard  to 
the  origin  and  descent  of  animals.  Some  scholars  have  sup- 
ported the  idea  of  (1)  special  creation  (i.e.,  that  all  species 
of  animals  were  originally  created  as  they  now  are),  but  since 
Lamarck  and  Darwin  there  have  been  an  increasing  number 
of  those  who  believe  (2)  that  living  types  of  animals  did 
not  originate  as  they  are  today  but  have  been  transformed 
while  descending  from  remote  ancestors.  The  evolution 
theory,  then,  supports  the  principle  of  "  descent  with  modi- 
fication" as  opposed  to  that  of  special  creation. 

Among  scientific  men  evolution  is  no  longer  on  trial,  but 
is  accepted  as  a  fact.  Throughout  this  book  such  a  point 
of  view  has  been  taken  for  granted  and  it  may  now  be  well 
to  examine  more  critically  the  facts  and  theories  on  which 
present  ideas  of  evolution  rest.  In  this  connection  there 
should  be  no  confusion  between  what  is  known  and  what  is 
believed — that  there  has  been  an  evolution  scientists  do  not 
doubt,  but  there  has  been,  and  is,  much  difference  of 
opinion  as  to  why  and  how  changes  have  taken  place.  After 
accepting  the  (1)  facts,  and  evolutionist  must  still  puzzle 
over  the  (2)  means  by  which  new  species  have  come  into 
being,  and  why  there  is  (3)  adaptation  to  the  environmental 
conditions  in  which  animals  live. 

EVIDENCES  OF  EVOLUTION 

Palaeontology  and  Geographic  Distribution. — As  was 
stated  in  the  last  chapter,  the  earliest  stages  of  the  earth's 

342 


EVOLUTION  AND  HEREDITY  343 

fossil  records  are  lacking,  but  as  far  back  as  they  can  be  read, 
they  clearly  indicate  that  the  simplest,  or  most  primitive, 
animals  appeared  first.  Though  many  primitive  types 
often  persisted  for  long  ages,  there  has  been  in  general  a 
progressive  change  to  new  and  often  improved  forms  which 
better  met  changing  conditions  of  environment.  In  spite 
of  the  imperfection  of  the  fossil  record,  there  are  a  number 
of  indubitable  cases  where  series  of  stages  which  follow 
each  other  in  order  are  known  (horse,  elephant,  etc.). 
Thomson  and  Geddes  say,  for  example:  "  What  seems  clear 
is  this,  that  in  early  Eocene  times  there  lived  small  five- 
toed  hoofed  quadrupeds  of  generalized  type — that  the  de- 
scendants of  these  were  gradually  specialized  throughout 
long  ages  along  similar  but  by  and  by  divergent  lines,  that 
they  lost  toe  after  toe  until  only  the  third  remained,  that 
they  gained  longer  necks,  more  complex  teeth  and  larger 
brains.  So  from  the  short-legged  splay-footed  plodders  of 
the  Eocene  marshes  there  were  evolved  light-footed  horses 
running  on  tiptoe  on  the  dry  plains. " 

Modern  animals  for  the  most  part  fall  readily  into  a  small 
number  of  phyla,  which  are  rather  sharply  set  off  from  each 
other.  Only  a  few  types  have  come  down  from  the  past, 
but  there  are  some  fossils  (Archseopteryx,  Fig.  102)  which  fill 
in  certain  gaps  and  indicate  what  the  evolutionary  changes 
have  been.  We  also  have  living  a  few  "links,"  like  Peripa- 
tus  (Fig.  33)  and  the  duckbill  (Fig.  107)  which  suggest  re- 
lationships that  would  have  remained  unsuspected  in  their 
absence.  Palaeontology  tells  a  story  which  is  fragmentary 
but  may  be  interpreted  only  in  the  light  of  evolutionary 
theories,  and  the  story  has  no  events  that  seriously  inter- 
fere with  such  an  interpretation. 

Groups  of  modern  animals  in  many  instances  are  known 
to  have  had  their  past  evolution  in  localities  where  they 
are  now  abundant.  Each  continent  has  certain  types  that 
are  characteristic,  both  as  fossils  and  as  animals  now  living. 
Furthermore,  groups  which  are  known  to  have  originated 
in  rather  recent  geological  ages  have  limited  ranges  of 


344  GENERAL  ZOOLOGY 

distribution  when  compared  with  those  of  ancient  lineage. 
Such  facts  lead  to  the  conclusion  that  specific  types  ori- 
ginated in  certain  parts  of  the  earth  (centers  of  origin)  and 
slowly  spread  to  other  favorable  localities.  Furthermore, 
it  is  well  known  that  particular  animals  which  have  been 
isolated  (on  islands,  in  valleys  surrounded  by  high  moun- 
tain ranges,  etc.)  develop  into  types  which  differ  in  certain 
characteristics  from  those  that  are  free  to  wander.  For 
example,  Australia  has  long  been  separated  from  other 
continents  and  has  been  subjected  to  comparatively  slight 
climatic  changes;  its  animals  are  primitive  and  show  few 
close  affinities  with  those  of  other  continents.  The  fauna 
of  the  Galapagos  Islands,  while  consisting  of  recent  types, 
is  composed  largely  of  unique  and  peculiar  species. 

Generalizing  from  the  facts  of  palaeontology  and  the 
geographic  distribution  of  animals,  it  may  be  said  that 
generalized  types  preceded  and  gave  rise  to  those  of  special- 
ized structure,  that  particular  species  arose  in  certain 
localities  and  gradually  spread  to  others,  that  species 
isolated  and  thus  prevented  from  mixing  with  others  are 
usually  peculiar  and  rather  closely  adapted  to  local 
conditions. 

Ontogeny. — The  "Law  of  Biogenesis"  asserts  that 
"ontogeny  repeats  phylogeny"  (page  60),  and  in  general 
the  evidence  for  evolution  from  the  development  of  indi- 
vidual animals  conforms  with  the  sequence  shown  by 
stratified  fossil  records.  All  vertebrates,  for  example,  pass 
through  a  fish-like  stage  (Fig.  115);  palaeontology  shows 
that  fishes  were  the  first  vertebrates  to  appear  on  the  earth. 
From  such  facts  the  older  evolutionists  reasoned  that  at 
one  time  there  were  only  fish-like  vertebrates,  and  that 
these  later  gave  rise  to  other  more  specialized  types.  This 
view  has  now  been  modified  somewhat  and  at  the  present 
time  the  same  facts  are  generally  interpreted  as  meaning 
that  all  vertebrates  are  related  because  they  pass  through 
similar  stages  in  development.  The  "fish-like"  stage  in 
the  development  of  a  mammalian  embryo  has  gill  slits  and 


EVOLUTION  AND  HEREDITY 


345 


its  limb  buds  are  somewhat  like  fins,  yet  such  facts  do  not 
necessarily  mean  that  mammals  were  fishes  at  one  stage  in 
their  racial  development. 

Though  the  ontogeny  of  a  particular  animal  usually 
repeats  the  general  sequence  of  descent,  the  record  may  be 
warped  or  distorted  by  events  which  did  not  occur  in  racial 
development,  or  it  may  fail  to  agree  in  other  respects. 
There  are  cases  of  " larval  adaptation"  in  which  a  develop- 


fish 


So/amander 


Frog 


Turtle 


Chicken 


Man 


FIG.  115. — Showing  stages  in  the  development  of  vertebrates.  The  fish  and 
salamander  have  functional  gills  in  the  last  stage,  but  all  other  animals  repre- 
sented lose  them  in  earlier  stages. 

ing  animal  possesses  certain  structures  (placenta  of  mam- 
mals, Fig.  107;  anal  filament  of  young  crayfish,  Fig.  30) 
which  could  not  have  been  present  in  mature  free  living 
ancestors  when  in  a  comparable  stage  of  evolution.  In 
other  instances  certain  stages  which  probably  existed  in 
ancestral  types  have  been  dropped  out.  In  such  "  accelera- 
tion in  development"  the  record  is  not  complete.  It  is 


346  GENERAL  ZOOLOGY 

also  probable  that  in  the  ontogeny  of  any  animal  there  is 
wide  discrepancy  as  regards  the  proportion  of  time  between 
various  embryological  stages  and  comparable  stages  of  the 
ancestors  when  in  similar  condition.  Despite  such  imper- 
fections in  the  ontogenic  narrative,  the  development  of 
animals,  when  taken  with  other  evidence,  gives  valuable 
support  to  the  theory  of  evolution. 

To  quote  Thomson  and  Geddes:  "It  must  be  admitted 
that  the  recapitulation  doctrine  has  been  often  stated  in 
somewhat  crude  and  exaggerated  form,  so  that  many  saving 
clauses  are  necessary.  The  human  embryo  is  never  like  a 
little  fish  or  a  little  reptile;  the  resemblance  is  between 
embryonic  stages.  The  recapitulation  is  general,  not  exact; 
there  is  often  abbreviation  and  a  masking  of  the  old  by 
the  new.  .  ..  .  It  must  also  be  frankly  admitted  that 
we  are  apt  to  get  into  a  vicious  circle  in  arguing  about 
recapitulation.  We  infer  the  pedigree  from  the  develop- 
ment, and  then  say  that  the  development  recapitulates  the 
pedigree.  .  .  .  But  when  all  is  said,  there  remains  good 
reason  for  keeping  firm  hold  of  this  idea — that  'ontogeny, 
or  the  development  of  the  individual,  is  a  shortened 
recapitulation  of  phylogeny,  or  the  evolution  of  the  race." 

Morphology  and  Classification. — If  animals  are  classified 
on  a  structural  basis  without  regard  to  evolutionary  theories, 
they  arrange  themselves  so  that  those  which  are  genetically 
related  come  in  the  same  group  and  a  sequence  is  established 
which  has  evolutionary  significance.  Studies  in  classifi- 
cation and  cataloguing  first  suggested  evolution  to  Lamarck. 
At  the  present  time  systematic  zoologists  use  all  the  evi- 
dences of  evolution  in  attempting  to  establish  more 
" natural"  systems.  Modern  classification  is  founded  on 
genetic  relationship  as  judged  by  ontogeny,  phylogeny, 
morphology,  and  physiology.  A  correct  system  of  classi- 
fication is  also  a  genealogical  tree. 

Morphology,  of  itself,  furnishes  good  evidence  for 
evolution.  The  presence  of  useless  vestigeal  organs  (man's 
appendix,  whale's  teeth)  is  hard  to  explain  except  as  a 


EVOLUTION  AND  HEREDITY  347 

survival  of  structures  which  at  one  time  in  racial  history 
were  functional.  The  principle  of  "homology"  has  also 
been  of  value  hi  supporting  evolutionary  theories.  For 
example,  all  vertebrates  have  the  same  series  of  bones 
arranged  in  the  same  way  in  their  limbs,  but  there  is  great 
variation  in  proportions.  It  is  difficult  to  explain  why 
such  diverse  organs  as  a  frog's  leg,  a  whale's  flipper,  a 
bird's  wing,  and  a  horse's  foreleg  should  show  homology, 
unless  it  is  assumed  that  all  vertebrates  came  from  a  com- 
mon ancestral  stock. 

Physiology  and  Ecology. — There  are  many  facts  in 
physiology  and  in  the  relationships  of  animals  that  are 
readily  interpreted  by  evolutionary  theory.  The  functional 
activity  of  the  nervous  systems  shows  steady  progress 
from  the  simpler  to  more  complex  animals.  Starting  in 
sponges  with  separate  and  independent  effectors,  simple 
reflexes  are  established  through  receptors  and  effector  cells, 
there  is  combination  and  coordination  of  such  reflexes  by 
adjusters,  and  finally  the  more  or  less  complete  domination 
of  primitive  effectors  and  receptors  by  a  central  nervous 
mechanism. 

It  is  hard  to  believe  that  such  intricate  relations  as  exist 
between  parasite  and  host  have  been  thus  from  the  begin- 
ning of  things.  If,  however,  we  assume  that  a  liver  fluke 
(Fig.  74)  was  first  a  free-living  aquatic  worm  somewhat  like 
a  planarian  (Fig.  73),  that  it  gradually  took  up  a  parasitic 
life  during  succeeding  generations  and  developed  the 
ability  to  live  and  multiply  during  larval  stages  in  the 
abundant  nourishment  provided  by  a  snail's  body,  the 
case  is  much  more  understandable. 

Experimental  Evolution. — To  any  thoughtful  person  the 
proof  of  evolution  would  seem  to  lie  in  experiments  and 
many  attempts  have  been  made  to  secure  positive  evidence 
in  this  way,  but  until  the  past  few  years  there  has  been 
little  that  is  satisfying.  Recent  investigators  have  con- 
tributed some  valuable  facts  as  to  how  animals  vary  and 
how  particular  hereditary  characters  are  transmitted  from 


348  GENERAL  ZOOLOGY 

generation  to  generation.  "In  this  case  and  throughout 
all  consideration  of  '  evidences/  it  must  be  remembered 
that  the  evolution  idea  cannot  be  logically  demonstrated. 
It  is  not  a  simple  induction  from  particulars,  thoroughly  as 
particulars  support  it.  It  is  a  way  of  looking  at  the  becom- 
ing of  things;  and  it  is  the  only  scientific  modal  interpreta- 
tion that  has  been  suggested.  It  is  a  formula  that  fits  the 
facts,  and  all  the  facts  it  fits  are  its  '  evidences. "  The 
most  recent  discoveries  resulting  from  experimental  methods 
will  be  discussed  after  the  chief  evolutionary  theories  have 
been  stated. 

THEORIES   OF  EVOLUTION 

Though  ancient  Greek  scholars  (Aristotle,  Lucretius) 
and  others  in  later  times  (Buffon,  St.  Hilaire)  had  vague 
evolutionary  ideas,  the  formulation  of  the  two  leading 
modern  theories  was  due  to  Lamarck  and  Darwin. 

Lamarck's  Theory. — In  the  author's  own  words— 
"Changes  in  environment  bring  about  changes  in  the 
habits  of  animals.  Changes  in  their  wants  necessarily 
bring  about  changes  in  their  habits,  the  new  habits  involve 
the  use  of  new  parts,  or  a  different  use  of  old  parts,  which 
results  finally  in  the  production  of  new  organs  and  the 
modification  of  old  ones."  The  central  idea  of  this  theory 
is  the  cumulative  transmission  of  functional  modifications- 
use  or  disuse  makes  a  particular  part  of  an  animal  increase  or 
degenerate  through  successive  generations.  Lamarck  fur- 
ther affirmed,  "Nature  preserves  everything  that  she  has 
caused  the  individual  to  acquire  or  lose  by  the  influence  of 
the  circumstances  to  which  the  race  has  been  for  a  long  time 
exposed,  and  consequently  by  the  predominant  use  of  a 
certain  organ  (or  in  consequence  of  its  continued  disuse). 
She  does  this  by  a  generation  of  new  individuals  which  are 
produced  with  the  newly  acquired  organs.  This  occurs, 
provided  that  the  acquired  changes  were  common  to  the 
two  sexes,  or  to  the  individuals  that  produced  the  new 
forms." 


EVOLUTION  AND  HEREDITY  349 

There  is  no  doubt  that  the  use  of  particular  parts  makes 
them  larger  or  more  effective  and  that  disuse  makes  them 
poorly  developed  and  comparatively  ineffective.  The 
blacksmith's  arm  is  strong;  the  banker's  sensitive  finger 
tips  detect  a  spurious  coin  at  once;  the  sedentary  business 
man  cannot  run  like  a  savage,  and  the  provincial  peasant 
cannot  acquire  broad  knowledge.  The  chief  weakness  in 
Lamarck's  theory  lies  in  the  fact  that  such  "acquired 
characters"  are  not  known  to  be  transmitted  in  heredity. 
The  banker's  children  are  as  easily  deceived  by  bad  money 
as  was  the  banker  before  he  began  his  training;  the  son  of 
the  blacksmith  can  have  powerful  arms  only  by  constant 
exercise.  In  spite  of  this  difficulty  the  theory,  while  not  so 
successful  as  the  next,  still  has  many  warm  advocates. 

Darwin's  Theory. — Darwin's  conception  of  evolution 
was  that  all  animals  and  plants  are  subject  to  (1)  small 
chance  variations ;  thus  certain  individuals  have  a  slight  ad- 
vantage over  others  in  the  (2)  struggle  for  existence.  This 
leads  to  (3)  natural  selection  by  a  (4)  survival  of  the  fittest, 
and  in  time  to  the  origin  of  new  species.  Darwin's  theory 
is  commonly  known  as  "  Natural  Selection."  Though  it 
did  not  wholly  discard  Lamarck's  ideas  as  to  the  trans- 
mission of  acquired  characters,  selection  was  its  important 
principle.  It  has  been  modified  and  supplemented  some- 
what since  Darwin's  time,  but  its  chief  features  have  stood 
criticism  remarkably  well.  Briefly  stated,  the  principal 
objections  urged  against  natural  selection  are  as  follows: 
(1)  that  it  does  not  explain  how  variations  that  are  not  use- 
ful from  the  beginning  may  be  selected;  (2)  that  variations 
in  some  cases  at  least  are  most  apt  to  take  place  when  the 
struggle  for  existence  is  not  severe;  (3)  that  it  will  not  ac- 
count for  the  perfection  in  detail  which  many  structures 
have  attained  (e.g.  the  wings  of  insects  that  are  remarkably 
like  leaves,  etc.) ;  (4)  that  many  apparently  useless  struc- 
tures have  developed  along  progressive  lines;  (5)  that  the 
range  of  variation  is  not  modified  by  selection;  (6)  that 
chance  variations  cannot  be  the  sole  materials  on  which 


350  GENERAL  ZOOLOGY 

selection  must  work  to  produce  new  types.  These  and 
others  that  might  be  cited,  however,  in  no  way  invalidate 
the  theory  as  a  whole,  and  it  is  looked  upon  by  the  majority 
of  scientific  men  as  the  best  working  hypothesis  we  have. 

The  following  concrete  illustration  may  serve  to  compare 
the  differences  between  the  two  theories.  From  palseon- 
tological  evidence  it  is  known  that  the  modern  horse  de- 
scended from  ancestors  which  had  several  toes  on  each  foot. 
According  to  the  Lamarckian  hypothesis  the  one-toed  con- 
dition probably  came  about  through  use  and  disuse.  In 
order  to  run  more  swiftly  horses  stood  on  the  tips  of  the 
toes;  the  middle  digit  was  used  most  and  hence  grew  larger 
through  successive  generations;  the  toes  on  either  side 
dwindled  away  with  disuse.  A  Darwinian  explanation  of 
the  same  facts  would  suppose  that  ancestral  horses  varied 
among  themselves;  that  those  with  larger  middle  toes  were 
able  to  run  faster  and  hence  survived  more  often  in  the 
struggle  for  existence;  that  the  middle  toes  were  progress- 
ively larger  and  the  side  toes  smaller  because  such  condi- 
tions better  fitted  the  environment.  According  to  the 
first  explanation  the  habit  of  using  the  middle  toe  and 
neglecting  those  on  the  side  led  to  a  one- toed  condition; 
according  to  the  second,  the  same  end  was  attained  by  the 
selection  by  the  environment  of  variations  which  made 
horses  better  fitted  to  survive  in  the  struggle  for  existence. 

VARIATION  AND  HEREDITY 

Since  the  scientific  world  accepted  the  facts  of  evolution 
and  had  theories  as  to  how  the  transformations  came  about, 
attention  has  naturally  been  directed  to  discovering  how  ani- 
mals actually  do  vary  and  just  how  characteristics  are  trans- 
mitted from  one  generation  to  another.  Variations  have 
been  carefully  studied  by  statistical  methods;  thousands 
of  individual  animals  have  been  measured  and  compared. 
Tests  have  also  been  made  by  breeding  experiments  to 
determine  how  particular  characteristics  are  transmitted 


EVOLUTION  AND  HEREDITY  351 

from  parent  to  offspring.  Though  all  the  problems  have 
not  been  solved,  the  chief  kinds  of  variations  have  been 
determined  and  some  fundamental  laws  of  heredity  have 
been  worked  out. 

Fluctuating  Variations  and  Mutations.— Some  character- 
istics vary  according  to  the  laws  of  chance  on  either  side  of 
an  average  condition.  For  example,  the  height  of  man  is 
about  five  feet  eight  inches  and  it  has  been  determined  by  a 
large  number  of  measurements  that  the  farther  men  depart 
in  stature  from  this  average,  the  fewer  they  are  in  numbers. 
There  are  many  men  whose  height  is  five  feet  seven  inches  or 
five  feet  ten,  but  those  that  measure  seven  or  five  feet  are 
rare.  Such  departures  from  the  average  of  a  species  are 
called  indeterminate,  fluctuating,  or  " chance"  variations. 
Sharply  separated  from  these  are  those  variations  which 
show  no  gradations,  but  are  clearly  differentiated  in  each 
individual.  The  short-legged  Ancon  sheep  appeared  sud- 
denly in  stock  which  had  previously  possessed  legs  of  normal 
type.  When  Ancons  are  bred  with  other  sheep  the  off- 
spring have  long  or  short  legs;  there  are  no  intermediate 
conditions.  Such  new  variations  which  hold  their  own  in 
heredity  are  commonly  called  mutations. 

Orthogenesis. — Where  variation  proceeds  in  a  particular 
direction  through  succeding  generations  it  is  said  to  be  ortho- 
genetic.  Eimer  was  the  first  to  point  out  that  many  varia- 
tions do  not  follow  the  laws  of  chance  but  run  in  particular 
grooves.  This  discovery  has  been  of  great  value  in  the 
understanding  of  certain  points  which  were  not  readily 
explained  by  natural  selection,  such  as  cases  of  mimicry 
of  one  species  by  another  and  the  very  elaborate  color 
patterns  developed  by  some  species.  It  makes  the  broad 
differences  between  groups  of  animals  more  easy  to  under- 
stand: e.g.,  why  the  insects  always  have  three  pairs  of 
legs;  the  spiders,  four;  and  the  vertebrates,  two. 

Somatic  and  Germinal  Variations. — Weismann  has  made 
a  distinction  between  variations  which  occur  in  the  bodies 
of  metazoans  and  those  which  influence  their  germ  cells. 


352  GENERAL  ZOOLOGY 

According  to  him  the  former  are  not  inherited  but  the  latter 
may  lead  to  marked  changes  in  succeeding  generations. 
Weismann's  Germ  Plasm  theory  lays  the  whole  responsi- 
bility for  heredity  on  the  germ  cells;  they  are  the  "  vehicle 
of  inheritance"  (Fig.  116).  The  body  (soma)  of  each 
individual,  though  carrying  the  germ  cells,  is  to  be  looked 
upon,  from  this  point  of  view,  only  as  a  by-product  of  them. 
Each  generation  of  germ  cells  produces  enough  soma 
(which  includes  all  muscle,  nerve,  gland — even  the  mind 
and  consciousness)  to  enable  it  to  survive  in  the  struggle 
for  existence.  But  the  germ  cells  control  every  detail  of 
the  body,  and  must  be  modified  before  there  can  be  heredi- 
tary changes  in  the  species  as  a  whole.  Nothing  in  the 

-Body 


FIG.  116. — Diagram  illustrating  germinal  continuity.  Through  a  series  of 
divisions  a  germ-cell  gives  rise  to  a  body,  or  soma,  and  to  new  germ  cells.  The 
latter,  not  the  body,  give  rise  to  the  next  generation.  (From  Guyer's  Being 
W ell-Born,  Copyright,  1916.  By  special  permission  of  the  publishers,  The 
Bobbs-Merrill  Company.) 

body  can  be  inherited.  All  somatic  qualities  are  caused 
by  some  modification  of  the  germ  cells,  which  reproduce 
all  the  " hereditary"  qualities  by  which  particular  species 
are  recognized  in  each  generation.  The  soma  dies  at 
the  end  of  each  generation  but  there  is  no  break  in  "  con- 
tinuity of  the  germ  plasm." 

Blending  Inheritance. — Animals  produced  from  a  zygote 
formed  by  the  union  of  two  germ  cells  partake  equally 
from  each  parent  in  many  of  their  characteristics.  A  large 
rabbit,  for  example,  when  bred  with  a  small  one  usually 
produces  offspring  of  intermediate  size.  In  many  such 
cases  of  supposed  blending,  however,  the  succeeding  gener- 
ations do  not  retain  the  intermediate  or  blending  condition 
but  are  separable  into  several  groups.  The  increasing 
number  of  such  " exceptions"  has  led  some  investigators  to 


EVOLUTION  AND  HEREDITY 


353 


believe  that  there  is  no  such  thing  as  the  perfect  blending  of 
hereditary  characters. 

Mendel's  Law.  —  There  are  many  qualities  which  do  not 
"  blend"  but  are  inherited  as  "unit  characters"  —  that  is, 
they  are  present  or  absent,  or  show  one  of  two  alternative 
conditions  without  intermediates.  Such  cases  follow  Men- 
del's  Law,  which  supposes  (1)  unit  characters  which  are 
(2)  usually  dominant  or  recessive  and  which  are  (3)  in- 
herited according  to  the  laws  of  chance.  Dominant  and  re- 
cessive characters  may  be  present  in  a  single  individual 
but  in  such  instances  the  former  usually  masks  the  latter 


FIG.  117. — Diagram  showing  the  scheme  of  inheritance  in  guinea  pigs  when 
black  and  albino  forms  are  crossed.  (From  Guyer's  Being  Weil-Born,  Copy- 
right, 1916.  By  special  permission  of  the  Publishers,  The  Bobbs-Merrill  Co.) 

so  that  only  the  dominant  character  is  shown.  In  other 
cases  the  unit  characters  may  produce  an  apparent  " blend" 
in  the  first  hybrid  generation  but  later  separate  out  and 
occur  in  later  generations  as  they  were  in  the  original 
parents.  Guinea  pigs,  for  example,  may  possess  a  colored 
coat  or  be  albinos  with  white  hair,  and  pink  eyes.  The  pig- 
mented  condition  is  dominant  to  the  albino.  Therefore, 
if  a  black  guinea  pig  (Fig.  117)  is  crossed  with  an  albino  the 
offspring  in  the  first  generation  will  all  be  black.  These 
individuals  are  hybrids  which  possess  both  black  and  albino 
qualities,  but  show  only  the  former  because  it  is  dominant. 


354  GENERAL  ZOOLOGY 

In  the  next  generation  (F2)  however,  the  characters  separate 
out  according  to  the  laws  of  chance  and  three  different 
classes  of  individuals  are  produced  in  the  following  ratios: 

1  BLACK:  2  BLACK  (ALBINO):  1  ALBINO 

The  first  and  third  classes  will  " breed  true."  If  an  in- 
dividual from  either  of  these  is  bred  with  another  like  itself, 
nothing  but  black  or  albino  offspring  will  result.  But  the 
"black  (albino)7'  hybrids  if  bred  together  (F2,  F3)  will 
always  separate  out  into  three  characteristic  classes — 1 
pure  black;  2  black  (albino);  1  pure  albino. 

Many  pairs  of  dominant  and  recessive  characters  are 
known.  Among  these  the  following  may  be  mentioned: 
brown  and  blue  eyes  in  man;  beardlessness  and  beard  in 
wheat;  rough  coat  and  smooth  coat  in  guinea  pigs ;  normal 
condition  and  peculiar  waltzing  condition  in  Japanese 
waltzing  mice.  In  each  of  the  instances  just  mentioned 
the  dominant  precedes  the  recessive  condition.  There  are 
other  pairs  of  characters  in  which  one  does  not  dominate  the 
other,  but  there  is  always  segregation  of  the  characters  in 
the  germ  cells  and  chance  for  recombination  after  fertili- 
zation. It  is  the  segregation,  or  separating  out  of  unit 
characters,  therefore,  which  is  the  important  thing  in 
Mendelian  inheritance. 

Bearers  of  Heredity. — In  heredity  particular  qualities 
are  not  passed  from  parent  to  offspring  but  something  is 
transmitted  which  determines  what  the  character  of  the 
offspring  shall  be.  A  guinea-pig,  for  example,  develops 
from  a  fertilized  egg  and  is  nourished  by  its  mother  during 
development.  There  is  no  white  or  black  hair  in  the 
egg  and  there  is  nothing  in  the  nourishment  supplied  by  the 
mother  that  makes  one  or  the  other  develop.  Nevertheless 
there  is  something  in  the  egg  that  makes  black  or  white  hair 
develop  from  the  material  supplied  to  the  growing  embryo. 
Evolutionists  have  given  up  the  old  idea  that  the  fertilized 
egg  contained  a  miniature  but  complete  animal  which  simply 
enlarged  its  parts  during  development.  There  is  not  only 


EVOLUTION  AND  HEREDITY  355 

transformation  of  food  material,  but  changes  in  structures 
as  well.  This  has  led  scientific  men  to  search  for  deter- 
miners of  one  kind  or  another.  Darwin  believed  that  there 
were  particles  (gemmules),  of  as  many  varieties  as  there  are 
categories  of  cells  in  an  organism,  which  migrated  to  all 
the  cells  during  development  and  determined  their  specific 
type.  There  were  serious  objections  to  such  a  view,  but 
other  evolutionists  have  postulated  somewhat  similar  deter- 
miners (micellae,  pangens,  etc.).  There  is  no  definite 
evidence,  however,  in  favor  of  any  such  arrangement. 

The  only  positive  evidence  that  indicates  bearers  of 
hereditary  qualities  points  chiefly  to  the  chromosomes. 
The  fact  that  the  chromosomes  are  divided  with  such  care 
during  mitosis  indicates  that  their  accurate  separation  is 
important.  Biologists  have  long  believed  that  this  was  an 
indication  that  they  were  the  carriers  of  hereditary  char- 
acteristics, and  during  the  past  few  years  Professor  Morgan 
believes  that  he  has  secured  evidence  by  breeding  fruit  flies 
(Drosophila  ampelophila)  which  enables  him  to  assign 
particular  characteristics  to  each  of  the  chromosomes  known 
to  be  present  in  the  germ  cells.  One  chromosome,  for  exam- 
ple, apparently  carries  the  factors  for  bent  wings  and  eye- 
lessness;  another,  those  for  banded  body,  dwarfing,  white 
color  of  head,  and  at  least  twenty  others;  the  two  remain- 
ing chromosomes  are  believed  to  carry  twenty-eight  and 
forty-seven  factors  respectively.  If  chromosomes  are  the 
sole  bearers  of  hereditary  qualities,  those  of  the  fruit  fly 
doubtless  carry  many  more  than  have  as  yet  been  assigned 
to  them.  But  Professor  Morgan  speaks  with  enough  assur- 
ance concerning  the  behavior  of  those  he  has  tested  by 
experiment  to  be  able  to  assign  characters  to  particular 
positions  on  the  chromosomes. 

CONCLUDING  REMARKS  ON  EVOLUTION 

Morgan*  says:  "Today  the  belief  that  evolution  takes 
place  by  means  of  natural  processes  is  generally  accepted. 

*  "A  Critique  of  the  Theory  of  Evolution,"  1916. 


356  GENERAL  ZOOLOGY 

It  does  not  seem  probable  that  we  shall  ever  again  have 
to  renew  the  old  contest  between  evolution  and  special 
creation. 

"But  this  is  not  enough.  We  can  never  remain  satisfied 
with  a  negative  conclusion  of  this  kind.  We  must  find  out 
what  natural  causes  bring  about  variations  in  animals  and 
plants;  we  must  also  find  out  what  kinds  of  variation  are 
inherited,  and  how  they  are  inherited.  If  the  circumstan- 
tial evidence  for  qrganic  evolution,  furnished  by  comparative 
anatomy,  embryology  and  palaeontology  is  cogent,  we 
should  be  able  to  observe  evolution  going  on  at  the  present 
time,  i.e.,  we  should  be  able  to  observe  the  occurrence  of 
variations  and  their  transmission.  This  has  actually  been 
done  by  the  geneticist  in  the  study  of  mutations  and  Men- 
delian  heredity." 

Delage  and  Goldsmith  in  their  "Theories  of  Evolution" 
(1913)  make  the  following  statements: 

"Two  important  phenomena  mark  the  evolution  of  the 
organic  world;  one  is  the  appearance  of  the  different  species, 
the  differentiation  of  the  various  groups  recognized  in  the 
classification  of  animals  and  plants,  the  increasing  com- 
plexity of  organisms,  their  evolution  from  the  lowest  to  the 
highest  forms;  the  other  is  the  adaptation  of  living  things  to 
the  conditions  and  necessities  of  their  environment.  The 
two  processes,  while  simultaneous,  differ  entirely  in  their 
natures  and  are  never  superimposed." 

Delage  and  Goldsmith  are  supporters  of  Lamarck's 
theory  and  therefore  attach  special  importance  to  the  trans- 
mission of  characters  acquired  through  environmental  in- 
fluences. Morgan  may  be  looked  upon  as  the  most  recent 
supporter  of  the  natural  selection  theory,  though  his  defini- 
tion of  "selection"  is  different  from  that  given  by  Darwin. 
To  the  Lamarckian,  adaptation  to  environment  is  of  equal 
importance  with  evolution;  to  the  selectionist,  adaptation  is 
the  natural  result  of  evolution.  There  is  no  question  that 
animals  have  followed  certain  more  or  less  arbitrary  lines 
during  their  evolution  on  account  of  the  inflexibility  of 


EVOLUTION  AND  HEREDITY  357 

heredity  and  they  have  also  shown  increasing  specialization 
to  adapt  them  to  environmental  conditions.  The  Lamarck- 
ians  maintain  that  animals  acquire  characters  in  their 
reactions  to  environment,  that  these  are  transmitted,  and 
hence  that  changing  environmental  relations  cause  evolution 
and  adaptation.  Darwinians  claim  that  there  has  been 
evolution  because  the  environment  has  selected  variations, 
and  that  there  is  adaptation  because  the  variations  which 
best  fit  the  environment  are  selected.  For  the  former  view 
there  is  little  or  no  evidence;  for  the  latter  there  is  increasing 
support  from  experiment  and  observation. 

Professor  Morgan  in  his  recent  book  proposes  a  some- 
what new  method  of  selection  as  the  basis  of  evolution.  He 
believes  that  selection  does  not  operate  on  small  fluctuating 
variations  but  on  numerous  small  mutations. 

"Evolution  of  wild  species  appears  to  have  taken  place 
by  modifying  and  improving  bit  by  bit  the  structures  and 
habits  that  the  animal  or  plant  already  possessed.  Evolution 
from  this  point  of  view  has  consisted  largely  in  introducing 
new  factors  that  influence  characters  already  present  in  the 
animal  or  plant.  ...  In  other  words,  the  emphasis  may  be 
placed  less  on  the  competition  between  the  individuals  of  a 
species  (because  the  destruction  of  the  less  fit  does  not  in 
itself  lead  to  anything  that  is  new)  than  on  the  appearance 
of  new  characters  and  modifications  of  old  characters  that 
become  incorporated  in  the  species,  for  on  these  depends 
the  evolution  of  the  race. 

"  The  mechanism  of  heredity  has,  I  think,  been  discovered 
— discovered  not  by  a  flash  of  intuition  but  as  the  result  of 
patient  and  careful  study  of  the  evidence  itself.  With  the 
discovery  of  this  mechanism  I  venture  the  opinion  that  the 
problem  of  heredity  has  been  solved.  We  know  how  the 
factors  carried  by  the  parents  are  sorted  out  to  the  germ 
cells.  The  explanation  does  not  pretend  to  state  how  fact- 
ors arise  or  how  they  influence  the  development  of  the 
embryo.  ...  So,  I  repeat,  the  mechanism  of  the  chromo- 


358  GENERAL  ZOOLOGY 

somes  offers  a  satisfactory  solution  of  the  traditional  problem 
of  heredity." 

There  is  perhaps  some  question  as  to  whether  hereditary 
characters  may  be  modified  by  selection.  That  is,  whether 
selection  in  a  given  direction  may  cause  variation  in  the 
same  direction.  The  bulk  of  the  evidence,  however,  ap- 
pears to  indicate  that  such  is  not  the  case.  According  to 
Morgan,  then: 

"  Evolution  has  taken  place  by  the  incorporation  into  the 
race  of  those  mutations  that  are  beneficial  to  the  life  and 
reproduction  of  the  organism.  Natural  selection  as  here 
defined  means  both  the  increase  in  the  number  of  individuals 
that  results  after  a  beneficial  mulation  has  occurred  (owing 
to  the  ability  of  living  matter  to  propagate)  and  also  that 
this  preponderance  of  certain  kinds  of  individuals  in  a 
population  makes  some  further  results  more  probable  than 
others.  More  than  this  natural  selection  cannot  mean,  if 
factors  are  fixed  and  not  changed  by  selection." 

The  question  of  evolution  must  be  left  in  this  state:  the 
fact  that  there  has  been  an  evolution  is  proved,  the  methods 
by  which  evolution  takes  place  have  during  the  past  few 
years  become  pretty  well  known,  but  the  causes  of  variation 
and  heredity  are  as  yet  largely  unknown. 


INDEX 


INDEX 


Acanthocephala,  188 

Acarina,  131 

Acquired  characters,  349 

Acridinae,  76,  77 

Adaptation,  36 

Adephaga,  114 

^Epyornis,  271 

Agassiz,  11 

Albatross,  272 

Alligator,  266 

Alternation  of  generations,  168, 

183,  188 
Amitosis,  42 
Amnion,  288 
Amoeba,  135 
Amphibia,  246 
Amphioxus,  228,  231 
Amphipoda,  58,  59 
Ancestor  of  arthropods,  45 
Anemone,  172 
Annelida,  203 
Ant  lion,  101 
Antelope,  295 
Anthozoa,  172 
Antimere,  17 
Ants,  119 
Ape,  293,  294 
Aphids,  96 
Aphis  lion,  101 
Apoda,  246 
Aptera,  69 
Arachnida,  123 
Archseopteryx,  270 
Archseornithes,  269 
Aristotle,  6 
Armadillo,  291 
Arthrodires,  330 
Arthropoda,  45 
Artiodactyla,  295 
Ascaris,  184 
Asteroidea,  192 
Aves,  269 


Bark  lice,  72 
Barnacle,  58,  61 
Bats,  291 

Bearers  of  heredity,  354 
Bears,  292 
Beaver,  292 
Bed-bug,  100 
Bee,  119 
Beetle,  113 

Bilateral  symmetry,  17,  177 
180,      Binary  fission,  137,  144,  145 
Biogenesis,  60,  344 
Biramous  appendage,  45,  57 
Bird  lice,  72 
Birds,  269,  280 
Bittern,  272 
Bladder  worm,  181 
Blending  inheritance,  352 
Body  cavity,  154,  183,  186 
Book  lice,  72 
Bot-fly,  110 
Brachiopoda,  190 
Brachycera,  109 
Brain,  312 
Brittle  starfish,  199 
Bryozoa,  191 
Bufo,  251 
Bug,  95,  98 
Butterfly,  103,  104 

Caddis-fly,  102 
Carabidae,  114 
Carbohydrate,  30 
Carnivora,  292 
Cats,  292 
Cattle,  295 
Caudata,  246,  250 
Cecidomyiidse,  109 
Cell,  15,  37 
Cell-division,  40 
Cell  theory,  43 
Centipede,  65 
361 


362 


INDEX 


Cephalization,  156,  177,  228 

Cephalochorda,  228 

Cephalopoda,  220 

Cercaria,  180 

Cestoidea,  180 

Cetacea,  296 

Chsetopoda,  203,  208 

Chalcid-fly,  118 

Chelonia,  264 

Chilopoda,  65 

Chiroptera,  292 

Choanocyte,  157,  158 

Chordata,  224 

Chromatin,  39 

Chromosomes,  40,  42 

Cicada,  98 

Ciliates,  142 

Circulatory  system,  286,  302 

Civilization,  323 

Clam,  217 

Classification,  13,  15,  346 

Clavicornia,  115 

Cleavage,  153 

Closed  blood  system,  206,  215,  238 

Cockroach,  72 

Ccelenterata,  163,  172 

Coelom,  18,  152,  154,  186 

Coleoptera,  113 

Colloid,  29 

Conjugation,  137,  144,  145 

Coral,  172 

Crab,  58,  59 

Crayfish,  50 

Cricket,  74 

Crinoidea,  202 

Crocodilia,  257,  265 

Crustacea,  50,  57 

Ctenophora,  173 

Cuckoo,  273 

Culicidse,  106 

Cuvier,  10 

Cyclops,  58,  61 

Cyclostomata,  231 

Cypris,  58,  61 

Damsel-fly,  70 
Darwin,  10,  349 
Decapoda,  59 


Deer,  295 

Definitions,  5 

Dermestidse,  115 

Devil-fish,  220 

Didelphia,  289 

Digestion,  300 

Diploblastic,  16 

Diplopoda,  64 

Dipnoi,  235 

Diptera,  106 

Diseases,   111,    139,    140,    141,    179, 

181,  186,  304 
Dobson-fly,  101 
Dogs,  292 

Domesticated  birds,  284 
Dragon-fly,  70 
Duck,  273 
Duck-bill,  289 
Dugong,  296 
Dytiscidae,  114 

Earthworm,  18,  204,  224 
Echinodermata,  192 
Echinoidea,  200 
Ecology,  6,  347 
Edentata,  291 
Elasmobranchii,  232 
Elephants,  295;  fossil,  336 
Embryology  of  man,  307 
Endoskeleton,  18 
Enteropneusta,  226 
Enzymes,  300 
Ephemerida,  69 
Eumeces,  258 
Eutheria,  289 

Evolution,  6,  342,  348,  355,  358 
Excretion,  35,  303 
Exoskeleton,  18,  19 

Fsoces,  35 
Fasciola,  179 
Ferments,  300 
Fertilization,  153 
Fishes,  235,  242 
Fission,  137,  144 
Fissipedia,  292 
Flagellates,  139,  140 
Flame  cell,  176 


INDEX 


363 


Flat  worms,  174 

Fleas,  112 

Fluctuating  variations,  351 

Flukes,  178 

Fly,  106,  109,  111 

Fossil  reptiles,  267 

Fossils,  328 

Frogs,  255 

Galen,  7 
Gall-fly,  117 
Gall-gnat,  109 
Gastropoda,  213,  216 
Gastrula,  152,  153,  165 
Geographic  Distribution,  343 
Germ  layers,  16,  153 
Germinal  variations,  351 
Gill  arches,  224,  225 
Gill  slits,  224,  225 
Glochidium,  218 
Goose,  273 
Gordiacea,  187 
Grasshopper,  73,  76,  79 
Grebe,  272 
Gull,  273 
Gyrinidae,  114 

Hagfish,  231 
Harvey,  9 
Hawk,  273 
Hemiptera,  95 
Heredity,  354,  357 
Hermaphrodite,  150,  177,  207 
Heterocera,  103 
Heteromera,  116 
Heteroptera,  98 
Hibernation,  253 
Hirudinea,  211 
Holothuroidea,  201 
Homoptera,  96 
Hook-headed  worms,  188 
Hookworm,  186 
Horse,  295 
Horse-fly,  109 
Horsehair  worms,  187 
Horses,  fossil,  339 
Huxley,  12 
Hydra,  16,  18 


Hydrophilidae,  115 
Hydrozoa,  163 
Hymenoptera,  116 

Infusoria,  142 
Insecta,  66 
Insectivora,  291 
Isopoda,  58,  59 
Isoptera,  72 

Kangaroo,  291 
Katydid,  73 
Kidneys,  230 
Kiwi,  271 

Lamarck,  348 

Lamellibranchiata,  217 

Lamellicornia,  115 

Lamprey,  231 

Lancelet,  228 

Law  of  Biogenesis,  60,  251,  255,  344 

Law  of  Recapitulation,  60,  251,  255, 

344 

Leech,  16,  211 
Lemurs,  293 
Lepidoptera,  102 
Lice,  95 
Life,  25 
Life  cycle,  35 
Linnaeus,  9 
Living  substance,  28 
Living  things,  33 
Lizard,  258,  261 
Locust,  73,  76,  79 
Loon,  272 
Lumbricus,  204 

Malaria,  108,  141 
Mammalia,  286 
Man,  293,  294,  297 
Manitee,  296 
Mantis,  74 
Mantle,  213 
Marsupial  pouch,  288 
Marsupial  ia,  290 
Marsupium,  218 
Mastigophora,  139 
Maturation,  151 


364 


INDEX 


May-fly,  69 

Mechanism,  30 

Medusa,  17,  163,  165,  168,  170 

Melanopus,  79 

Mendel's  Law,  353 

Mesozoic  Age,  332 

Metabolism,  34,  35 

Metagenesis,  168,  169,  171,  172 

Metamerism,  16,  156,  203,  208,  222 

Metamorphosis,  68,  104 

Metapleural  folds,  228 

Metazoa,  147 

Midge,  108 

Migration,  bird,  283 

Migration,  fish,  244 

Milliped,  64 

Mind  of  man,  311 

Mite,  131 

Mitosis,  40 

Mole,  291 

Mollusca,  213,  222 

Monkey,  293 

Monodelphia,  289 

Monotremata,  289 

Morphology,  6,  346 

Mosquito,  106 

Moth,  103,  105 

Moulting,  93 

Mud-puppy,  247 

Muscidse,  111 

Mussel,  218,  220 

Mutations,  351 

Myriapoda,  64 

Natural  selection,  11,  349 
Necturus,  247 
Nematocyst,  166,  167 
Nematoda,  4,  183,  186 
Nematoidea,  183 
Nemertina,  182 
Neornithes,  269,  270 
Nests,  bird,  284 
Nests,  fish,  243  . 
Neuroptera,  100 
Nutrition,  298 

Obelia,  163 
Octopus,  220 


Odonata,  70 
(Edipodinse,  76,  77 
(Estridse,  110 
Oligochseta,  210 
Ontogeny,  344 
Ontogeny  of  metazoa,  151 
Onycophora,  63 
Open  blood  system,  81,  206 
Ophiuroidea,  199 
Opossum,  290 
Organic  substances,  30 
Organs,  149,  155,  169 
Origin  of  earth,  327 
Origin  of  life,  25 
Origin  of  man,  314 
Orthogenesis,  351 
Orthoptera,  72 
Ostracoderms,  329 
Ostrich,  272 
xOwl,  273 
Oyster,  194,  218,  219 

Psedogenesis,  250 

Paired  appendages,  45,  47,  57,  128, 
228,  229 

Palseozoology,  5,  340 

Parazoa,  160 

Parthenogenesis,  150 

Passeres,  274 

Pasteur,  27 

Pearls,  219 

Pedicellaria,  195 
\  Pelican,  272 
^Penguin,  271 

Pentadactyl  limb,  229 

Perch,  235 

Perching  birds,  274 

Peripatus,  63 

Perissodactyla,  295 

Petromyzon,  231 

Phalangida,  130 

Philidota,  293 

Phyla  of  animals,  19,  21,  24 

Physa,  213 

Physiology,  6,  347 

Phytophaga,  115 

Pigs,  295 

Pinnipedia,  292 


INDEX 


365 


Pisces,  235 

Placenta,  288 

Placoid  scale,  232,  233 

Planaria,  16,  174 

Planesticus,  274 

Platyhelmia,  174 

Plecoptera,  71 

Pliny,  7 

Poisonous  snakes,  263 

Polychaeta,  210 

Polymorphism,  72,  119,  165,  169,  172 

Polyp,  163,  165,  166,  170,  172 

Porcupine,  293 

Porifera,  157 

Primates,  293 

Proboscidia,  295 

Proglottid,  180 

Protective  coloration,  84,  283 

Protein,  30 

Protoplasm,  28 

Prototheria,  289 

Protozoa,  134,  145 

Quaternary  age,  336 

Rabbit,  292 

Radial  symmetry,  17,  19 

Radula,  214,  217,  221 

Hat,  293 

Rays,  232 

Recapitulation  Theory,  60 

Recognition  colors,  91 

Redi,  26 

Redia,  179 

Regeneration,  53,  86,  159,  176,  196, 

200,  202,  206,  240,  248,  305 
Rejuvenescence,  146 
Reproduction,  36,  145,  150 
Reptilia,  257,  266 
Respiration,  67,  68,  82,  302 
Retrogressive    metamorphosis,     62, 

228 

Rhinocerus,  295 
Rhopalocera,  103 
Rhynchocephalia,  257 
Rhynchophora,  116 
Robin,  274 
Rodentia,  292 


Rotifera,  188 
Round-worms,  183 
Ruminant,  295 

Salamanders,  250 

Salientia,  246,  251,  255 

Sarcodina,  135 

Sauria,  257,  261 

Saw-fly,  116 

Scale  insects,  97 

Scallop,  223 

Scolex,  180 

Scorpion-fly,  102 

Scorpionida,  130 

Scyphozoa,  170 

Sea-cucumber,  201 

Sea-squirt,  226 

Sea-urchin,  200 

Seal,  292 

Sense  organs,  79,  80,  205,  206,  244 

Serpentes,  257,  261 

Serricornia,  115 

Seta,  203 

Sharks,  232 

Ship  worm,  219 

Shrew,  291 

Silkworm,  105 

Siphonaptera,  112 

Sirenia,  296 

Skeleton,  18,  19;  vertebrate,  229 

Skink,  258 

Sleep,  303 

Sloth,  291 

Slug,  16 

Snail,  213 

Snake,  262 

Snake  star,  199 

Snipe,  273 

Society,  320 

Soma,  140,  145,  148,  352 

Somatic  variations,  351 

Songs,  bird,  283 

Specialization,  47 

Spicules,  158 

Spider,  123,  124,  129 

Sponge  fishery,  161 

Sponges,  157,  158,  169 

Spontaneous  generation,  25 


366 


INDEX 


Sporocyst,  179 
Sporozoa,  141 
Squamata,  257,  261 
Squid,  220 
Squirrel,  293 
Starfish,  192,  197 
Stegocephalians,  331,  332 
Sting  ray,  234 
Stone-fly,  71 
Stone-lily,  202 
Stork,  272 
Symmetry,  197 
Symphyla,  65 

Tabanidse,  109 
Tapeworm,  180 
Tapir,  295 
Teeth,  234,  287 
Teleostomi,  235 
Teredo,  219 
Termite,  72 
Tern,  273 
Tertiary  Age,  334 
Testudinata,  257,  264 
Tettiginse,  76 
Threadworms,  183 
Thrips,  72 
Tick,  132 
Tipulidse,  106 
Tissues,  148 
Toad,  251 
Torpedo,  234 


Tortoise,  264 
Tracheae,  68 
Tree  frogs,  256 
Trematoda,  178 
Trichina,  187 
Trichoptera,  102 
Trilobites,  328,  329,  330,  332 
Trimera,  116 
Triploblastic,  16 
Trochophore,  190 
Tryaxalinae,  76,  77 
Tube-foot,  192 
Tunicata,  226 
Turbellaria,  174 
Turtle,  264 

Variation,  350 
Vertebrata,  228,  230 
Vesalius,  8 
Vitalism,  30 
Vorticella,  143 

Walking-stick,  74 
Walrus,  292 
Wasp,  118 
Weismann,  351 
Whale,  296 
White  ant,  72 

Zoogeography,  5 
Zoology,  5 


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WILL  BE  ASSESSED  FOR  FAILURE  TO  RETURN 
THIS  BOOK  ON  THE  DATE  DUE.  THE  PENALTY 
WILL  INCREASE  TO  SO  CENTS  ON  THE  FOURTH 
DAY  AND  TO  $1.OO  ON  THE  SEVENTH  DAY 
OVERDUE. 


FEB  121933 


26  1940 

1 3  1941 


isi  m 

JUN 

JUN    8196813 


LD  21-50ni-8,-32 


376237 


LIBRARY 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


